Biblical view of Sex before marriage
Sexuality is a powerful and priceless form of human interaction.  We all know it is powerful , and its pricelessness is too often lost in our society.  When you love someone it is normal to desire to express that love sexually, and the desire to do so is typically very strong! 
It has to be powerful to have the potential to be such a beautiful bond and communicator.  And, like everything else that is powerful it is also potentially dangerous, too.  As with most things powerful, guidance is needed for its proper use if we are going to avoid destroying as much or more than we create.  Naturally, thus, the Bible speaks (very openly and not at all squeamishly) about sex. 
In the Bible, sex is powerful.  It is therefore sometimes wonderful and sometimes ugly.  Like a fire, in its place, sex is inspiring; outside its place, it is devastating.  Sex is a strongly encouraged blessing between a husband and wife, and the marriage bed is pure.  At times when I read about it there, I get the sense that sex between a husband and wife is worshipful of sex’s creator – God (see Ezek 34).
I was recently asked to defend biblically the idea that sex before marriage is sin.  If you are reading this but you already believe that sex before marriage is sin, then realize this is written as a defense.  If you are carrying guilt for former or current sexual behavior, then feel free to skip to the end.  In regards to the question, though, here are my thoughts on the matter, and what I think the Bible teaches:
Is sex before marriage a sin?  I am not sure that we can make an air-tight case for saying “The Bible says that sex before marriage is sin.” Because that exact phrase does not appear.  However, this is largely because the terms in the New Testament are broader – more general – than merely “sex” – sexual penetration.   The New Testament also does not say “serial homicide is sin” – but murder is, even murderous rage (Matt 5:21-22).  If murder is sin, human driven genocide is probably assumed.  If any sexual impurity is sinful (see verses below), then sexual intercourse outside of the relationship for which it was created and commanded in Genesis, would also be assumed.   Let’s look at the Greek for a second.
One of the clearest teachings about impurity is found in Paul’s letter to the Thessalonians.  For example,
“…For this is the will of God, your sanctification; that is, that you abstain from sexual immorality…” (1 Thess 4:3, NASU)
The word here in this passage is from the root word <pornos> – obviously the root word for our “porno-graphy” (Lit. “sexually immoral graphic representation”).  It is a word used to reference all kinds of sexual bad-behavior.  In many translations, it is translated into “fornication.”
The International Standard Bible Encyclopaedia notes that “in the New Testament, with both the literal and the figurative application, we find porneia, and porneuo (Matt 5:32; 15:19; John 8:41; Acts 15:20; 1 Cor 5:1; 6:13,18; 7:2; 10:8; 2 Cor 12:21; Gal 5:19; Eph 5:3; Col 3:5; 1 Thess 4:3; Rev 2:14,20-21; 9:21; 14:8; 17:2,4). The intensive ekporneuo = “to be utterly unchaste” is found in Jude verse 7. Every form of unchastity is included in the term fornication.”
So, how could we be sure that in biblical terms, this would refer to sexual behavior between an unmarried man and woman?  Would this apply to two young people, who, dating only one another and seriously in love, want to have sex?  Maybe the writers and early Christians just didn’t consider this kind of sex as “immoral?”  Sorry, but it just doesn’t hold water.  Paul uses these ideas in the main passages that talk about this which is found in 1 Cor chapters 6-7.
In these passages, Paul clearly says that the reason that people should marry is because they can’t keep avoiding having sex!  Paul is clearly talking about how he thinks it is better for someone not to marry at all… however, if you “burn with passion,” then you better go ahead and get married… and when you do, make sure you don’t deprive one another.  Only the thickest person would miss that the implication is that if you are not married, you should not be having sex.  Otherwise, why make marriage for those who cannot keep from having sex? (1 Cor 7:1-8) Also, consider the added truth that husband’s and wife’s bodies belong to one another.  Would this change just because you are unaware of who the husband and wife will be someday?  I don’t know, but I think it is worth considering.  Further, I think the command to Timothy to treat unmarried girls as “sisters” probably does not include a sexual relationship with them (1 Tim 5:1).
Jesus clearly teaches that lust (the desire to possess something that does not belong to you, to meet your own desires) is sin.  Even just lust is sin equal to adultery! (Matt 5:27-28)
So, if desiring to have sex with someone is equated to adultery, then what about actually having sex with them!  Further, in order to defend that sex outside of marriage would be ok, you would have to make an argument that you could have intercourse without lusting…hmmm.  So, just impurity with a woman not your wife is sin.  Just seeking sex with her is sin, and the book of wisdom (Proverbs) clearly teaches that following after a woman for sex is the path to death (chapters 5 & 9).
Obedience is all the reason we need.  If God draws boundaries, we should respect them whether we understand, agree, or want to.  If He loves us, then what He seeks for us is, in the end, the best.
The strongest key, however, is to make note of the fact that Jesus calls us to love one another and seek the best for one another – having sex with an unmarried person is not what is best for them – the issues that it creates are monumental.  OK, so some part of this may be reaching for an answer to “if God loves me, then why would he forbid sex before marriage?”  And asking questions about “why” can be dangerous.
Please follow that we are seeking to understand better, we are NOT trying to hold God accountable for His motivations.  We don’t have to understand, but maybe we can see evidence of why God would want to draw boundaries around sex.  If we are convinced that God loves us, then we can assume that His boundaries are meant for our best – are evidence of His love… and thus if we choose otherwise, then we must somehow not be loving people involved.  Does the evidence match?  What does it mean that sex before marriage is not God’s best?
Here are some of the more obvious issues:
The first is the most obvious: the risk of damaging consequences… like single  parenthood, temptation for abortion, STD’s – the percentage of which is a little overwhelming these days (I recently read 25% of teens, for example), and all the consequences of these too.
There is a great opportunity for regret… is it universal?  I personally think that eventually, probably, even if it isn’t immediate.
It virtually guarantees a sense of comparison… what kind of insecurity does it create to be compared to other lovers?  Plus, under the next heading, how do you feel about being compared to former (and what about FIRST) lovers?  Do we ever forget our first?  This is no curse, just a real consequence.
It is probably hypocritical.  A great way to measure our honest convictions is to dealwith how we feel about things closer to home.  It is easier to objectify someone we don’t know.  Would we want to encourage our sisters, daughters and future    wives to engage in the same behavior?  Would we want them to?  What about the same for future husbands, brothers, or sons?
If we would not want someone to
1.  teach our loved ones that sex is the main form of value that they have, or
2.  encourage that belief, or even
3.  behave in such a way as to combat that belief…
then we have no business doing so with someone else’s sister, son, or future spouse either.  To engage with someone else’s daughter what we would not desire for our own daughter is raw hypocrisy.  To engage in some behavior that we would not be happy about our future spouse doing is the same.  If I love someone I want the best for them, and am willing to pay appropriately to see them have it.  I am convinced that sex before marriage is not best, even if it weren’t sin!
For those who would say that they wouldn’t care if their sister, etc. were doing the same thing, then I would appeal to the creator argument.  If Col. Colt told you about the proper use for a revolver or if Henry Ford had some ideas about your Model T, then it would be wise to listen to them, as the designers.  If God has words on something He designed, like sex, then we should assume He knows more about it, its strengths, weaknesses, opportunities and threats, than we do.  For us to think we know more is serious arrogance and maybe delusional.
Much less, having sex with someone other than your spouse (even before you know them) is certainly NOT loving your future spouse!  God’s grace does not  call us merely to “shalt not,” but to “shall.”  It calls us to purity and holiness and maybe especially, LOVE.  Not having sex until marriage is the most loving choice for you, the person you are with, your future spouse, and their future  spouse – and maybe others (your parents, your future children, etc?)
It creates and/or encourages a question of what the person is valued for.  This is actually one of the best hidden, but most dangerous consequences of premarital sex.  More often than is probably known, women especially engage in sex in order to either keep a guy around, to feel approved of, or just to feel loved in some way.  There is a great and poignant movie clip about this in the movie The Fisher King    (http://www.youtube.com/watch?v=4qo1Oi8lwtA)             So many people don’t feel desired at all, and the closest they feel to desired is used.  We want to feel welcomed in as part of the family like an adopted child or a puppy; however if we don’t think we can have that, we will settle for being the chicken to be devoured at dinner, if that is as close as we can get.  Do we ever want to risk communicating that to another human?  How can a follower of Jesus (or Jehovah, or even The Buddha, for that matter) take the chance of promoting that belief in anyone?
You ask:  but what if the other person knows what they are doing?  First, is it possible they do, if they are doing it despite the consequences? I see this as akin to someone selling an early DC comic online for $1… would they really sell it for so   cheap if they knew its value?  Second, even if they do buy into this value of short- term performance only, are we going to be party to that belief, or will we stand against it?  I will stand against it.  Compare this to God’s design.  Once someone has committed to spend the rest of their life devoted to you, under God, then you never have to fear whether your performance (or lack thereof) is going to be what   makes you valuable, or worthless, to the spouse.
I once had a kind lady tell me in counseling that a rule life had taught her was:  “if I am willing to have sex with a man, then I never have to sleep alone.”  What an amazing reversal of God’s best:  “When you know you will never have to sleep alone again, then you should have sex with that man.”
Human value is something I am very passionate about – mainly because I think God is passionate about His artwork… His <poema> (Eph 2:10)… His craftsmanship.  We are precious and valuable mainly because we are His and the works of His Hands.  We are treasures to almighty God – the perfect judge of value and art.  Maybe I will write more about this later, but I am truly cautious to try to not come across as offended as I am, in the Spirit, by anything that devalues another human.
We are truly precious in His sight, and He has made us by His Hands, and purchased us with His Blood.  We are His sheep, though lost; He comes and finds us and His angels party over the sinner found.  He emptied Himself, took on what it means to be a man and a servant to the point of death on a cross – through the power of the Spirit, out of obedience for His Father and out of Their love for us.  What could possibly prove more that we are treasure?  The way we deal with sex in our culture is primarily about de-valuing people, it seems to me… when it was created to communicate a life-long value already determined in a covenant that was created to be a living parable of how God loves us!
With this in mind, I have to assume that sex before marriage is disobedience, which makes it sin… it is offensive to God’s best design, which must be sin… and it is a sin against one’s own body (you literally create a debt to yourself).  Sexual immorality is sin.  And, as with most sin, it seems that God calls sin what is not only offensive to Him, but also what is less than His best for us.  So often, it seems that He is trying to guide us in protecting ourselves even from ourselves.  As Pogo the Possum once said “We have met the enemy, and he is us” and we all know what he meant!  God desires to save us from all sin and death, once for all… starting now (and going on forever).
Meaning, you can be free to choose to live in God’s best for you – one aspect of which is to avoid sexual immorality (1 Thess 4:3) and instead commit to your future gift/sex life with your future spouse.  Also, accept the free gift of God’s graceful forgiveness.  I think we know that we don’t manufacture purity – even if we have somehow managed to avoid pre-marital sex.  We, in our own flesh, are still dressed in filthy rags, spiritually.   Surely no one thinks that we can somehow take our impurity and by impurely working harder somehow purify ourselves?
You can’t purify something by washing it with (metaphorically) dirty water and filthy rags.  We need someone pure.  Of all men, only Christ is pure.  By the same token, there is nothing so impure that it can resist the purifying power of the blood of Christ.  His love cannot be broken, His power to cleanse is peerless and perfect.  You can be made pure, no matter.
Only in Christ could we be pure…
Only in Christ are we pure…
and in Christ we ARE pure… how could we not be?
Here is what makes sense to me:  no matter whether we live or have lived in the truth of our purity or not, those in Christ are pure.  No matter what mistakes we have made in the past, Christ makes His bride pure.  Follow Christ; accept His free gift of salvation (just ask); abide in Him, and you must be pure in Him… and then (and only then) can we have the freedom to live a life worthy of that calling.
In other words, in Christ, and only in Christ, we are pure, because we are in Him and He is pure; our behavior cannot change our identity – cannot make us more or less pure.  We can only behave more or less purely.  We are not slaves… though we can act like slaves.  We cannot become any more pure than we are in Christ.  So, embrace the purity in Christ and live accordingly.  It is for freedom that you have been set free.   It works God’s way.