I've heard that it's something to do with hormones - that after sex, testosterone levels drop quite suddenly? I mean, right after sex, it's very difficult to achieve an orgasm for a second time (for guys) and so there isn't really much point in them actually having the same interest in girls that they usually would.
In my opinion, it's perfectly natural for guys to feel less interested in girls straight after sex, even if they are completely in love with them.
I mean (sorry to put it like this) but when a guy is looking at a hot in a porn magazine, and he starts masturbating, is it not true that after he's finished, she doesn't actually seem so hot anymore? But then she becomes hot again a little while later?