Bah, video games are silly, aren't they? I know that many games did the whole 'play as a male or female' thing just fine, but in some games, it wouldn't make much sense. Sure, a girl could close shut the jaws of Oblivion, and she could just as well be in the SAS with some craaaaaaaaaaaazy nickname like Soap or...I dunno, hairdye, but don't you think it would look rather awkward for a woman to be walking around in, say, Gears of War, wearing half a car?
Maybe that's just me, but it seems like that sort of armor would have to be heavily modified to the point of actually wrapping around the woman's body and directly fitting the breasts and whatnot so that you'd be able to tell she was female at all. I don't mean this in an insulting way, because, by all means, women are perfectly capable of wearing power armor. It's just that power armor is usually BUILT in a way that it accentuates the fact that the guy wearing it is REALLY BUFF. That wouldn't fit well on a female, it would just look awkward, and if they built it in a way that accentuates the fact that she's female, the guys who made the game would get yelled at by annoyed people everywhere for the girls having breasts that are overly large(because we all know how most game designers are.)
Point is, we have to draw a line somewhere between male leads and female leads. While the whole both genders thing would be great in many games, it just wouldn't work in quite a few.