I could give a long post about how I have a dysmorphic disorder and I'm so gaunt and preoccupied about my weight IRL, I choose female toons because males in WoW are awkwardly animated and big boned.
Then, I could also honestly tell you that I roll females just because from time to time, it is so ridiculously enjoyable to convince someone they just got floored by a girl in a video game. It never gets old, with all the e-peeners in WoW or the attitude that chicks suck at video games. Seems stupid, but believe me .. some guys freak. Especially in public duel zones.