Well, a good reason is this: I am male. I know what it is to be male. If I am playing a RPG, I typically want to role play as something I am not. Thus, playing a female is just a logical step.
I tend to find that when I play a male character I stay more true to my real self, and I do not explore the role playing to its limits. Essentially, my male characters are typically similar to me: When I play a male character, I typically view my choices as "What would I do?". My female characters, though, run the whole gamut between angelic princesses and cold blooded assassins who care for nothing more than their paycheck. In general, the gender disparity allows me a larger degree of disjointedness between my personality and the personality of my character.