(ready for people to yell at me)...
Cause of insecurities AND the fact that (I'm speaking for men here, FYI) society has set it up that a man's social value in the eyes of his male friends is raised exponentially when he has sex with a chick. You get rewarded for it, it makes you feel good, so you make sure others know when you hit it. Pintoca asked why this has to do with insecurities - cause a truly secure person would not need to seek acceptance based on something like this. My college buddies used to literally get fucking pissed at me cause I didn't take part in the morning ritual of talking about the girl you fucked the night before. I just never saw the need. Now, don't get me wrong, I'd make sure people knew if I hit the skins with a real hot chick, but looking back on it I am ashamed at that behavior. It wreaks of approval-seeking