Assumption is the mother of all ****-ups. I never understood why guys always assume that anyway. The fact is that there are actually just as many female gamers out there as there are male.
Being female, I generally prefer playing a female character, but I always play both genders just to get the most out of a game.
Although I find the feature important because I like being able to choose my gender, I would still buy a game even if the gender was fixed.
Fable is a good example for that. I missed the option to play female, but it didn't stop me from playing it, even if it wasn't that good a game. <_<
I think most guys just play female characters because they enjoy watching them walk around in their underwear.