For me, two things happened once I had sex with a girl.
As I look back on it, I can say that they happened literally every time, although I was unaware of these dynamics at the time.
I know a newlywed couple who have sex less than once a month because of this -- he doesn't respect her, she knows it, and she doesn't trust him, so she doesn't want to give herself to him.
It's very sad, and more common than you might think.
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.
When I was in college, I remember having an experience that I referred to as a "love hangover." After being with a girl, the next morning I always felt an emptiness.
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?
And the guy -- who doesn't treasure his wife because of the sex before marriage -- still wants sex but not as a total bonding experience with his wife. And consequently they'll have better and more frequent sex because they respect each other more and love each other more deeply.
Sex is a mysterious thing that causes a deep bond between people, even if we call it "casual." The problem is this: the more I bond with other girls, the less I'll be able to bond with my future wife.