I agree with coffeesp00ns -- A binary language just doesn't leave enough room for variety. One transgender relative of mine has taken to signing their email with both m and f names.
One of the interviews, I think with Diane Sawyer, mentioned Deuteronomy 22:5 which prohibits cross-dressing. One of them says, "You see how far back it goes. They wouldn't prohibit it if it wasn't already happening."