I'm a guy and I play as girl characters whenever the option to do so is present. They look better for a start, and I like watching virtual females kick ass. There's something about watching a muscly man go around killing things which is a little unappealing to me, I'd much rather watch a woman do it. And no I don't care if she's portrayed "sexily" or not, in fact I usually like it when developers DON'T bung on the fake "sexiness" with the subtlety of a jackhammer. Sometimes subtle is sexier anyway. Also I like roleplaying rather than immersing my own personality in a game, and I'd rather roleplay someone who DOESN'T have my own characteristics (that's what roleplay is after all - it's playing a role, not playing yourself). Making the avatar the opposite gender makes that a little easier to do.
I really recommend it to guys to play as girls once in while, just to see how differently you're treated in a multiplayer community. It's an educational experience. Play as a girl online convincingly and you'll quickly learn just how immature guys can be. Even if you do NOTHING to provoke anybody other than calling your avatar Sally instead of Sam, you'll generally get a LOT more attention of both the good AND the bad kind. But mostly the bad kind. The amount of guys wanting to "cyber", or give me heaps of abuse, or just kill me because my avatar was female, have always far outweighed the ones who would give me free stuff.