As for having it built into the core... it would be such a small percentage I doubt it's even worth having it there. However custom profile fields are coming in 1.1, could always utilize that. And no, nor does anyone know when 1.1 will be out , but it will be soon
Not being flip, and no offense (nor display of ignorance meant), but don't the majority of transgender folks identify with one sex? Chaz Bono considers himself male, for example. Is there a segment of transgender folks who reject male/female distinction altogether?