One's gender doesn't necessarily have to correlate with sex.

You can think of a gender like a name: a self-assigned identifier to refer to someone as. A "Joe" is anyone who expresses a wish to be identified as "Joe", most commonly (but again, not always) by being assigned the name of "Joe" and being happy with it. Yes, it's theoretically a circular definition, though doesn't cause problems in the real world for the vast majority of people.

(Also "Adult {sex} human" is gramatically correct, not "Adult human {sex}")