Twice at the University of Utah, it was suggested in classes that I was in that waiting for sex until after marriage was sheer stupidity. Students said 'how could anyone leave something so important with a big question mark until after the wedding iteself? After all, sexual compatibility is important.' Or something to that effect.
Being a believer in Church of Jesus Christ of Latter Day Saints, it is my belief that God has commanded his children to have no sex outside of marriage between a man and a woman. This includes premarital sex (fornication), post-marital sex with other partners (adultery), sex by yourself (masturbation), and sex with others of the same gender (homosexuality).
I also believe that God commands things because they are right, not that they are right because he commands them. That being the case, there should be something demonstrably bad about breaking commandments. I think it should be obvious that we can't expect strong scientific evidence for everything that Heavenly Father says as soon as He says it, but I believe that over the long term, His commandments are for our good. And this is one example.
Science has shown that God is right in the case of not having sex before marriage, in purely temporal marital duration terms. For a believer, my claim that 'God is right' isn't very interesting... it's like saying the sky is blue. Even for the atheist or the agnostic (at least the well-educated ones) it isn't a surprise that "traditional belief systems evolve in a highly beneficial way to their adherents."
Many studies have shown that those that have sex before marriage have a somewhat higher risk of divorce. Apparently the group of those that have had sex before marriage is much larger than the group that hasn't.
So why the divorce rate difference? In my view, sex is like a bond. In a marriage, there are multiple relationship bonds or links. Living together, friendship, sex, experiences together, maybe intellectual similarities, personality similarities, etc. So more bonds is good. The person who doesn't have the sex and living together bonds, but still has enough relationship bonds to get married, gets those two more links for their relationship when they get married. More bonds, less divorce.
So that's one reason of mine on why God commands his children not to have sex outside of marriage. Of course it's not an all or nothing thing. It's not 100% divorce for premarital sex and 0% for the premaritally faithful. I can't find any of the actual studies online for free at the moment, but I'd guess there's only a few percentage points greater probability of divorce for those who don't wait.
The same research also shows some other interesting factors. Women who only had sex with the person they married had the same rate of divorce as those that waited until marriage. Women with multiple premarital partners obviously had a significantly higher rate of divorce.