Doesn't really make sense to do in team fortress, an already completed game. There's nothing wrong with more playable females (Valve's next major multiplayer title, Dota 2 for example has plenty of playable females.), but TF2's characters were designed with their silhouettes in mind to help discern character classes at a distance.
Plus, the "meet the" videos, in-game dialogue, and the online fandom has gone to GREAT lengths to characterize the playable classes' unique (though a little one-dimensional) and humorous personalities. Changing the existing characters' genders, or adding new characters which share the same class, plus new voice-overs seems like an unnecessary amount of work for this already released game.
In retrospect, maybe they could have made a couple of the existing classes female to begin with, and no personality would be lost, and diversity would be gained, but this seems like something to concern the development team on a future game, not TF2.