Maybe it's just built into "the system." But one thing's for sure: I'm not alone. I know many people having marital problems because they engaged in premarital sex.
They go into the marriage with lack of respect and lack of trust, two absolute necessities for the health of any marriage.
So you would imagine that having sex would have been completely fulfilling -- the crowning achievement in the worship of my "god." And yet, there was often a lack of fulfillment afterwards. Why is it that sex, if it's so important to me, leaves me with an empty feeling? I then concluded: "I just need more [sex], that's all." (We often think this way about stuff we hope will fulfill us, then doesn't.
For example, we get the car we've always wanted, but then it's just "okay" after awhile. There wouldn't be any "emptiness." I've found that girls often don't fully understand what's going on when it comes to sex.
And while something inside her is telling her it's the right thing to do, something inside the guy is telling him just the opposite, yet he proceeds. For the physical pleasure no doubt, but also, I think, for another reason: it makes him feel like a man.