I don't really care for the idea adding females if it's just for the sake of satisfying what I'm assuming is a very small subset of people playing the game. Not that I don't care about our fellow female gamers (quite the contrary), but if females are just randomly mixed in, I feel that that would be very immersion-breaking. Though I guess maybe having the option to have an all-female prison could work too, if only for the aesthetics of it.
Now what would be a cool idea would be to have female prisoners added to the mix (maybe add an option at the beginning?), with perhaps some subtle nuances in their AI that would make them act differently and have differing needs from their male counterparts, which could then add the challenge of dealing with separate male/female facilities and potential male/female interaction (sexual assault, rape, pregnancies, etc.). That being said, I feel like adding such "sensitive" subjects to the game would cause all the negative Nancy's to pour out of the woodwork, creating a steady stream of negative PR for the game and for Introversion. If Introversion is prepared to deal with and/or doesn't care about said negative publicity, I think it could add some cool dynamics to the game. If not, I won't really feel like the game is missing a whole lot.