Like it or not, living together (fornication) outside of marriage is a sin, and is a sin that God will not let go undisciplined (Heb. 13: 4) for those who take on the title of Christian (Heb. 12: 6) in vain (Ex. 20: 7).  Identifying oneself as a Christian and then willingly choosing to make the practice of ANY sin a lifestyle is the New Testament application of the ancient commandment (Ex. 20: 7).  
At the same time, fornication is not a sin that cannot be confessed and forgiven (1 John 1: 9), and forsaken (John 8: 11) for the glory of God and the benefit of those involved.  In most cases, it's simply a matter of choosing to live a life in accordance with the, "...mind of Christ - 1 Cor. 2: 16) or  choosing to live a life in accordance with an alternative "plan" that usually is designed to please one-self or the impulses of one's fallen nature (Romans 7).
It does not take the mind of a great theologian to know which "plan "(singular) is of God and which "plans" (plural) are endorsed by the devil.  The devil doesn't care which plan we choose or why, as long as it is not the plan that God has in mind.
Thinking that noting has happened yet was the great error of those to whom Noah (prior to the flood) preached (2 Pet. 2: 5).  Such rationalization as ,"everyone's doing it", and "nothings has  happened yet" has backfired in the faces of multitudes who choose to engage in activity that God has called sin.  Thank God that He usually (not always - Acts 5) allows a period of time between pointing out a sin and  sending judgment, giving us a chance to repent and avoid the impending circumstances.   Never-the-less, there comes a time when God has had enough and heads will roll!
Divine discipline can range in anything from living with a guilty conscience up to and including physical death (Acts 5). Such consequences refute the false teaching that the grace of God and the eternal security of the saved soul is a license to sin.
Living in any sin can greatly compromise one's usefulness to God.  (I say "any" sin because all sins equally ( James 2: 10) take the sinner out of fellowship with God).  We are focused on the sin of fornication as this is the subject of this lesson, but it is worth noting that the principle of being out of fellowship with God applies to ANY sin  When one is living in (any) sin, he/she is not in fellowship (2Cor. 13: 14) with God (Habakkuk 1: 13).  Church Age believers who are not in fellowship (close, abiding, intimate walk) with God rarely have God and/or the things of God high upon their daily "to do" lists, let alone choose to involve themselves in fulfilling their part in the fulfillment of the Great Commission.
In balance, Note that God can still use individuals to serve His purposes, even during periods of time when they are entertaining sin.  The fornicating woman of (John 4: 28-30) was used of the Lord to evangelize a whole village.  The woman caught in the act of adultery (John 8) was used of God to point our to her accusers their own sinfulness.   This did not mean that the Lord approved of the fornication of the woman at the well, or the adultery of the woman who was caught in the act. 
 For the advancing disciple of the Church Age, he/she will  desire to place and keep him/herself in a position where he/she can be best be used of God.  Accordingly, there is NO room for ANY known sin to be a part of one's lifestyle IF one wants to avoid divine discipline and maximize his/her potential in his/her post salvation spiritual life.
The forfeiture of  1Cor. : 12-15) rewards can involve some (2John 1:8) or all (1Cor. 3:15) of the rewards that the same believer could have otherwise received in addition to eternal life and the general environment of Heaven that ALL believers will experience.
Being content to forfeit one's rewards and accompanying privileges in Heaven in order to live out of fellowship here on Earth is a decision that one will deeply regret when the 1Cor. 3: 12-15 event takes place in Heaven.  For many believers, it will not be until then that one finally sees that how one's chooses to live here on Earth determines how that same believer will experience eternity.  The plan of God makes Heaven a place of equal opportunity for each believer to enter into discipleship and be rewarded for it.   However, even a passive look at the Christian Community will confirm the reality of (Matthew 7: 13, 14) in that only a minority of each generation of Man chooses to be saved, and only a minority of those saved ever enter into or remain in true discipleship.  The finding of the broad and narrow way can be applied to both salvation and discipleship.
Genuine disciples do NOT serve God in order to be noticed by Man or to be rewarded by God (Matthew 6:2).  More believers would choose to live for God and not for self if  they came to appreciate all that God did to make it possible for them to enter Heaven and not spend eternity in what the Bible calls the Lake of Fire (Rev. 20: 15).  Sine many believe its what they have done that qualifies them to enter  Heaven, they fail to  see or appreciate that it is what God has done that made it all possible.
Unbelievers have no choice but to be influenced and controlled by the "hostile (Romans 8:7)" fallen nature with which we ALL come into this world.  Once one is born again, he THEN has the option of being influenced and controlled by God the Holy Spirit.
When under the influence of the fallen nature, the individual will primarily live for what pleases one self.  When under the influence of God the Holy Spirit, one will primarily live for and what pleases God.  What  a disciple (Luke 14: 26)annot do is live for self, others, or other things AND GOD at the same time.  Many have tried, NO ONE has succeeded!
Fornication is nothing new.  Homosexuality is noting new. Lesbianism  is nothing new.  Bestiality is nothing new.  Polygamy  is nothing new.  Adultery  is noting knew.  Incest is nothing new. Prostitution is nothing new.  Child molestation is nothing new.  Sexual assault is nothing new.  The Inclusion of sexual acts in the "worship" rituals of  pagan religions is nothing new.  Many of these activities have been practiced since the earliest generations of Man.  
Depending on the culture involved, these  sexual activities have not only had an association with religious activity in the ancient world, but have been considered to be activities that range in everything from being worthy of capital punishment in some cultures up to  being given governmental approval via licensure on the other.   
In the secular realm, in societies where such activities are viewed as criminal offences, there IS the practical need to "rate" the seriousness of such acts and to impose an appropriate punishment for those convicted of engaging in such activity.   Most societies, governments, and individuals would not place fornication or adultery in the same boat as homosexuality or rape, but according to James 2:10, when someone violates any ONE of God's laws, he stands as guilty as the one who has broken them all!   Heterosexual fornication and/or adultery is no more and no less offensive to God than homosexuality, lesbianism, or bestiality. In divine viewpoint, sin is sin, and ALL sin repulsive to God (Hab. 1: 13).
Scripture, contrary to the "Sunday-School" image that many of us have of Biblical characters, reveals many of the negative choices that the Godly men and woman  of Scripture chose to entertain.    By "godly", I am referring to men and woman who identified themselves as believers in the God of the Bible.  Some of these individuals (e.g., David) were the (human) ancestors of the humanity of the Lord Jesus Christ.  Human rationalization points to the negative choices that spiritually and temporarily successful  individuals  have made when attempting to self-justify our own deviate sexual activities that run contrary to the applicable commands (not options) during the dispensation in which we live.  Justifying one-self on the basis of what someone or "everyone else is doing" was the fatal error of many who perished in the flood of Noah's day when God reduced the world's human population to 8 individuals.
Granted, the truth is the truth no matter who it is that is communicating  it.  The difference between the impact that such truth can make, however, is often contingent on the communicator's consistency in walking the walk as well as talking the talk.  This is why being in fellowship with God is so critically important if we are going to be in the BEST position to be used of God.  When we choose to live in or with a sin that we refuse to acknowledge, confess, and forsake, we are NOT only NOT in a position to be used of God, but we are in a very good position to be used of the devil!  In most cases, the day to day UNSPOKEN testimony of our daily lives has set or destroyed  our usefulness to God to minister to those we share time with in one activity or another, or with whom God brings into our personal periphery for THAT purpose!
In truth, everyone is NOT doing "it" (fornication/adultery).  The majority in any generation in many societies may be doing "it", but the majority in each generation will also choose to disbelieve the Gospel and wind up in  Hell.  The majority of believers will make post (after) choices that greatly reduce what their heavenly rewards could have otherwise been (1 Cor. 3: 15). Following the  lead from the majority (Matt. 7: 13) is seldom  the right choice concerning spiritual matters.
In order to "connect the dots" and see the big picture concerning why living in the sin of living together (fornication), we must begin by acknowledging  on what the Bible has to say about "living together".   For the purpose of this study, living together means to function in a sexually/intimate relationship outside of umbrella of Christian marriage.   Christian marriage is a moral and legal bond between a male and a female human being who CHOOSE to enter into a permanent relationship that is built on and functions under the principles laid out in Scripture.   Christian marriage addresses pre-marital, marital, and post marital behaviors.  Just because two persons who claim to be Christians get married does not mean that they were, are, or will function under the true principles of what God designed marriage to b.
Our primary Scripture verse, that clearly addresses the SIN of living together (fornication) verse is Hebrews 13:4 (NASB).  There we read, "Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge."  The simplicity and exactness of this verse offers no room for an alternate interpretation. 
The Title and content of the New Testament Book of Hebrews confirms that it was initially written  to early believers of the Church  Age who came from a Jewish background. It was included in the Canon of Scripture and is there for the edification of ALL Church Age believers.   The Church Age began in the approx. year 30-33 AD  (Acts 2) and will not end until the Rapture (1Thess. 4: 16, 17) takes place.  Like it or lump it, the commands of Scripture (and the believers' accountability) pertaining to the Church Age  do NOT diminish with the passing of time, as human viewpoint and the devil would have us believe.
Like a posted speed limit sign along a highway. we may not agree with it, but we cannot deny it's presence and can still be held accountable for it.
In time (1st Century AD) in which the Book of Hebrews was written,  "marriage" was understood to mean a moral and legal bond between a man and a woman.   There are many forms of marriage.  The form of marriage being referenced in Hebrews 13:4 is Christian marriage.   A Christian marriage is one that is based and functions in accordance with the relevant Church Age doctrines that address the subject.   How well or how poorly  believers  who entertain  Christian marriage is one of the areas under angelic observation (1Cor. 11:10) bringing glory to God or shame to the relationship on a daily basis!   The functions of the Christian husband/father, Christian wife/mother, and Christian children are commands, not suggestions!   As Heb. 13: 4 clearly established, pre-marital sex is forbidden.
The circumvention and/or rejection of marital authority opens the door to the rejection and circumvention of parental authority.   Satan, understanding the value of Christian marriage,  directs and ongoing assault on these fundamental pillars of Christian marriage.  To the devil's advantage, all of the persons involved have a fallen sin nature that is inherently "hostile (Romans 8:7)" to God and to the things of God.  This fallen nature is alive and well  in ALL unbelievers and in believers (Romans 7) when they are out of fellowship (2Cor. 13:14) with God.  When ANYONE is either spiritually dead, as in the case of ALL unbelievers, or out of fellowship, as in the case of believers living in or with ANY sin that has not been confessed/forsaken,  they are naturally receptive to all of the other alternative lifestyles and marriages that the devil promotes.  
In the subject matter at hand, these alternatives include other forms of marriage and/or  the rejection of the concept of marriage all together.  It is from this root that the fruit of "living together" has grown.
Scripture DOES not lay down ANY specific procedure that is to take place in order to establish and to begin a Christian Marriage.  The authors of the various religions, Christian or otherwise,  have all created their  own traditions as to what is required to get "married" in their own version of what the Church is.  Since there are NO physical altars or officiating priests authorized in Scripture for the Church Age, it can be argued that such persons and objects are the products of religion, and on that basis, are  NOT required (by God) for a man and a woman to be married in the eyes of God.  Virtually all secular governments have laws that establish when two people are married in the eyes of the Law, and have commissioned ordained clergy to be AMONG those who can marry two persons who have fulfilled the legal requirements (marriage license, etc.).  Scripture is clear, however, about what sexual behaviors are and are NOT to take place before, during, and after marriage. 
Regardless of what legal procedure(s) or religious rituals one engages in to establish a marriage, those who enter Christian marriage are answerable to God from whatever point both believes acknowledge each other as their spouse and THEN consummate the union through sexual intercourse.  In Christian marriage, the union is between a man and  woman.  Same gender marriage is NOT a "Christian" marriage.  To be a Christian marriage, the married couple  must establish it and function within it in compliance with the Word of God.  During the sexually active years of the human environment, sexual intercourse is a fundamental part of  Christian marriage.  Sexual intercourse between same-sex individuals is not possible without violating the Word of God.
Again, what the secular laws of Man require, prohibit, or endorse concerning the establishing and/or termination of marriage is not the same as what the Word of God has to say about these same issues.
Scripture is clear that fornication (sexual intercourse prior to "marriage" ) and that adultery (sexual intercourse between two people when at least one of them is married to someone else) violates the Word of God. 
Satan has had considerable success in attacking, altering, and redefining the family structure that God has established, beginning with the undermining of marital authority. 
It was by deceiving the woman, later named Eve, by implying  that there was something better (Gen. 3:5) for her and Adam than what God had in mind, coupled with Adam's desire to please his mate rather than obey God (Gen. 3:17) that set the stage for the FALL the entire human race.  The devil continues to offer his alternatives to God's design for Christian marriage knowing that his alternatives cater to the fallen nature in Man and appear offer the "benefits" of a relationship without any cost. God's way will always cost the advancing disciple something of a personal nature!  Placing people ahead of God on one's list of priorities is one of the things that disqualifies Church Age believers from either entering into or remaining in discipleship.
The stability of the family structure, as God designed it to be, and the stability of the society that the individual families collectively construct is one in the same.  Show me a society that once did, but no longer, honors the institution of marriage, and I will show you a society that is headed for the rocks.
The permissive will of God does allow individuals and societies at large to adopt alternative lifestyles, but NOT without consequences. The general population, consisting of believers and unbelievers alike, does have a lot to lose when alternate lifestyles and family structures become the "norm" and not the exception to the rule.  There is not a military, a domestic police force, or a system of "justice" that can provide national security for its people once the hand of God has moved against it in the name of divine (national) discipline.  
Nations are always in a state of  either spiritual advancement during which times the nation  receives the blessings of God in the midst of
everything else that is taking place, or it is in a state of spiritual decline in which it receives divine discipline in the midst of everything
else that is taking place.  The nation in moral decline may still exist, but will not offer the same environment that it did when it was being blessed by God.
When divine discipline is placed on ANY given nation, it will impact the lives of believers and unbelievers alike.  The cycles of national blessings and discipline usually take place over the span of several generations.  Accordingly, how much of any given soul's earthly lifespan is spent in the upward trend of  national blessing or the downward trend of national discipline is determined by one's birthdate and date of earthly departure.  Since  the date of one's birth and the date of one's departure (Psalms 31: 15) are in the hands of God, the individual can do little to change the ever-changing direction that a society is taking.  What each soul DOES have control (and accountability) over is how the decisions he/she makes to follow the crowd to take a personal stand when the society that he/she is in is in moral decline.
What does NOT change is the objectives of the Great Commission.  Souls still need to be saved, and saved souls need to be edified regardless of which direction the nation at large is heading in at any given time.
No nation, being governed by Man, is EVER going to be perfect, but God will only allow the rise of evil to go so far before the nation is subjected to divine discipline.  National divine discipline can be manifest by such things as a poor economy, inability to provide or maintain food supply, wide spread crime and violence, military defeat, disease, plagues, and natural disasters, just to name a few.   Each wave of destruction is greater in intensity that the preceding one until the issues that brought on the discipline are resolved or the nation is totally removed from the world community.
I had the pleasure of having been born in an environment of national blessings, and now share the concern of those are witnessing the moral decline of the American society.  God placed me here on Earth a few years after World War 2 (when America was the recipient of great national blessings).  Now in 2014, it is clear to anyone with an ounce of spiritual discernment that the USA is the recipient of divine discipline that will only get worse  if we, as a nation, continues along its current course.  The ONLY thing, I believe, that stands between the USA and the potential of  receiving the maxim divine national discipline (removal from the world's community of nations) is the degree in which many non-governmental people and organizations in the USA  are contributing to the last (Matt. 24: 14) wave of fulfilling the Great Commission, and  secondarily by the impact that USA foreign policy (unwittingly) plays in the fulfillment of END TIME prophecies to take (primarily) take place in the Middle East.
America's founding fathers had their own timing and reasons for establishing , maintaining, and promoting their agenda for the USA, and God (Acts 17: 26) has His!
Until the return of the Lord Jesus Christ, human viewpoint will always be the dominating mindset of the nations and world at large.  Human viewpoint is always received well by spiritually dead  individuals and believers who are out of fellowship with God.  In times of national divine blessing, human viewpoint will believe that it is because of his/their own accomplishments that things are "going well".  In times of national divine discipline, Man will blame God for the all SELF INDUCED misery that is taking place.
The challenge to the believer living in the midst of national discipline is that the more godly one lives here in the devil's world, the more isolated and persecuted the disciple will be. The challenge becomes even more intense as ungodly persons and nations are overtly blessed by devil (Luke 4:6) who flaunt their immorality in the faces of those who choose to honor God and are "no better off" for doing so. The "no better off" part is the devils lie, as those with spiritual discernment have learned that such "pleasures" and "liberty" is not worth what it when  compared to what it can cost them in the eternal state.
This principle applies to unbelievers  and  believers alike, as the pursuit and attaining of worldly prosperity and pleasure can blind those involved from recognizing the eternal consequences IF the pursuit and attaining of such things is given a greater priority than the things of God. God is NOT opposed to legitimate temporal prosperity UNLESS it becomes an impediment to one being born again (John 3:3) or to one's post salvation spiritual life (Rev. 3). It gains one NOTHING (of eternal value) if one gains the whole world (Mark 8:36), if it costs him an eternity in the lake of fire (Rev. 20: 15) as an unbeliever, or his/her heavenly rewards (1Cor. 3: 12:15) if he is a believer.
But what does all this have to do with living together?
NO post salvation sin can undo the regenerating work that God the Holy Spirit performs (Titus 3:5) at the moment of salvation.  This principle highlights the extent of the grace of God.
Never-the-less, we have seen there are negative consequences for believers who choose to entertain sin.   These consequences involve divine discipline here on Earth and the loss of reward (1Cor. 3: 12-15). 
Scripture (Hebrews ) makes it clear that living together is a sin.  When believers commit any  post (after) salvation sin, they remain saved but are out of fellowship (Habakkuk 1:13/2Cor. 14:13) with God until the known sin(s) are confessed (1John 1:9) to God in private prayer.  If the sin (such as the sin of living together) is one of lifestyle, then the sin must also be forsaken (John 8: 13), as well. 
The periods of time that ANY believer is out of fellowship with God can be as brief as the time it takes to confess (and if need be, forsake) the known sin(s), or can involved the rest of one's post salvation spiritual life here on Earth.  During these periods out of fellowship, the believer is not in a position to have God the Holy Spirit perform very much"divine" good through him (John 15:5), but IS in a position for the devil to nullify much of the impact that the same believer could have otherwise had here in his (the devil's Luke 4: 6) world. 
When I say the "devil's world, I refer to the extent in which God in His permissive will and for a divine purpose allows Satan and the Eph. 6: 12 forces of evil to function.  The daily headlines in the media are evidence of the devil's influence!
The production of divine good is what the post salvation spiritual life is all about, as this ( and NOT by the production of human good) is how the born again believer glorifies God.
When out of fellowship, the same believer can choose to live an otherwise relatively moral lifestyle, practice the futile rituals of "religion", and do all kinds of humanitarian work and human good, but NONE of these things are acceptable alternatives to the requirements of 1John 1:9/John 8: 11.  Until one returns to fellowship with God, the development of his/her post salvation spiritual life is on "hold" and will advance no further.
The quality of one's post salvation spiritual life what the BIG DEAL concerning the issue of living together (or any other sin) is all about.
Where any given believer chooses to place the identifying and executing of the TRUE post salvation spiritual life on his personal list of priorities is what determines how much of a big deal (if any) the issue of living together will be for him or her as an individual.
The natural curiosity and sex drive in human beings will dominate the thoughts and actions of most of us at one time or another.   Some will go as far as to make sexual immorality  a lifestyle.  Adding the teaching of Matthew 5: 27, 28 to the mix, one can see that that divine standard in much greater than the standards of Man's Law concerning adultery. The grace and mercy of God may allow some to elude the some of the temporal consequences (unwanted pregnancies, disease, death, divorce, shame, financial security, etc. of engaging in sexual sin, but of GREATER concern to the advancing disciple is his/her diminished usefulness to God and the increased usefulness to the devil. but one's ability to fulfill the purpose for which God created each one of us, that being  the glorification of God (Col. 1: 16) is on the line.  NEVER underestimate the unspoken testimony of your own daily life, the devil hasn't!
Post salvation sin does NOT result in the loss of salvation, but does open the door for divine discipline (Hebrews 12: 6) here on Earth and loss of reward(s) in Heaven (1Cor. 3: 12-15).  If one truly desires to learn of and to execute the TRUE post salvation life of discipleship, confessing (1John 1:9) and if need be, forsaking (John 8:11) sexual sins of thoughts and deeds (or any other form of sin) is NOT an option.  It is a requirement.
No matter how far we may wander, there is always a divine solution (1John 1:9) for the sin(s) we commit.  When we come to realize that we are ALL in the same James 2:10 boat, and therefore ALL equally in need of salvation, getting back onto the track of the straight and narrow way that leads to life can become a whole lot easier, no matter what the circumstances of our personal situation may be.   
If sexual sin is not a great temptation to you as an individual, then thank God for it.  If it is an area of weakness, know that there is power available to born again believers to handle this weakness than is greater than the powers that pressures you to entertain such thoughts and actions.
Man was created for the primary purpose of glorifying God.  During the dispensation of the Church Age, the human soul of a spiritually dead unbelievers glorifies God by choosing to believe in a presentation of the Gospel. Believers glorify God by learning and applying the relevant passages of Scripture to the minute by minutes events and circumstances of his/her daily life.   Regardless of what sin is involved, one cannot live in a lifestyle of sin without compromising the primary purpose for which God created him/her, which is to and glorify God at the same time. 
How much of a "big deal" living together (or ANY other sin) is reveals  where on one's list of priorities the glorification of God is placed.
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