I used to think you were supposed to wait... but then I became a teenager with raging hormones or just something to prove. So I waited a whole month or so. The next relationship in which I had sex... I wasn't even in the relationship yet. But I guess I don't have much experience with this as I've only had sex with 2 boyfriends *while* they were my boyfriend... this is probably why I don't have a problem having completely meaningless sex.. which may or may not be a good thing.
