That's something you won't see on TV or in the movies, but it happens a lot. The "love hangover" was a strange occurrence for me.
Mainly because when I was in college, sex was my "god." As a male, it's what I thought about morning, noon and night. If you have, you should stop and consider, "Why is that?
Deep down, she doesn't really enjoy being with him. Since "girls use sex to get love, and guys use love to get sex," a couple will have sex before marriage. The guy does it because he wants it even more than the relationship itself. Now that I'm out of college and many people around me are getting married, I'm seeing it happen all the time.
Then, after the marriage, the woman has what she wants: a commitment from the man. And, because she may be harboring resentment because he had sex with her before they were married, she is now not interested in sex. The antidote: waiting for marriage to have sex will give the man a greater respect for his wife and the woman a greater respect for her husband.
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.