Huh? I have dated several chicks waaaaaaay hotter than me. That's a win dude, what are you talking about? Just don't think about it. Just think about how lucky you are for landing them. You shouldn't think "Oh, I'm not good enough," but "god damn I'm lucky."
Besides, women don't judge appearance the way guys do. Attractiveness is a very important quality for men; for women, it's just a potential good quality. They look at the overall value of a man, not specifically how attractive they are. If they're attractive, then cool, if they're not but have some other good qualities, then awesome. Guys are much more centered on attractiveness.
So, really, while you might think they're out of your league, they might feel like they got a catch, because you have some other great qualities.