Reading through the comments thus far I feel like I am an anomaly.
I am male and I usually play as female characters, especially in fighting games and action games, because I feel it adds an extra level of aesthetic pleasure to what's occurring on screen. I'm a big fan of fantasy and style, and I personally believe women are far more interesting to look at when it comes to style due to the curvy shape associated with the average female body. Don't misunderstand my words, I'm not trying to be a snobby pervert who doesn't want to flat out say I enjoy looking at boobs, because that's not what I'm talking about (although I do enjoy looking at boobs and shapely behinds, especially when they are properly proportioned). What I mean with the curvy shape of the female body being more aesthetic pleasing in action scenes and the like is that motion and movement tend to look more fluid and imaginative, whereas male characters in fight scenes usually seem more grounded in their attacks and demeanors which is boring.
That said, I'll usually play as a male character in MMO's and such, probably because it feels natural to play that way. I say I am an anomaly particularly because I choose to play as female characters when they have character and depth to them, or when I feel I can give them more character and depth if I so desire.
TL

R
I prefer to play female because the games I play are more enjoyable with female characters. I play male the more blank slate the avatar is.