Well, first let me say that I don’t have to always understand God’s reasoning, in order to trust that He knows what He’s doing when He says something is or isn’t sinful. The simple fact that He says it is a sin… means it is, because God does not lie.
That said, I think we do know some of the reasons sex outside of marriage is wrong.
Sex outside of marriage is a sin because it is a lie.
At its most basic, it is selfish, not loving; it’s not about what is best for the other. It’s also bad for us spiritually, emotionally, and mentally (and can also be bad for us physically), partly because of what it does to distort our view of others and ourselves–it degrades our human dignity.
We human beings tend not to value those things (or those relationships) which do not require any sacrifice or “price” from us. So it also encourages us to place less value on the other person, and on the relationship.
Does this mean all married people value each other, and all sex within marriage is always holy, all of the time? Of course not. No one is making the argument that sex outside of marriage is bad because sex within marriage is always good. (That would be a rather illogical argument, anyway.)
But it does mean that
without marriage, there is
always a missing necessary component in sex. We can never reach the ideal in sex outside of marriage, no matter how much we try, no matter how much we want it, because there will always be that necessary piece that’s missing.
For someone who has not had awesome, fulfilling, married sex, this might be difficult to grasp. But for those who
have… no additional explanation is necessary.