The best explanation of female roles in games I've ever seen

[Professor S]

We have to keep in mind that the majority of people who play video games (as we know them) are men. When you play a game, you are inserting yourself into the character. That is why so many great video game protagonists are virtually devoid of personality (Master Chief, the guy from BioShock) or allow you to choose it (Mass Effect). You want to relate to your character, and if you are male, you'll choose the protagonist that best relates to you: The male. I would expect women who play games to choose the female.

Its not "sexist" in a traditional sense. Its about immersion.

Oh, and the fact that most designers make female protagonists look one step away from a hooker doesn't help either.