I thought this is why they started using "gender". "Gender" was used in place of "sex" so people could put down what they thought they were. Your "sex", on the other hand, cannot be changed, as it is your genetic structure. You are either "XX" or "XY". If they want to mess around with the definition of their imaginary psychological term, whatever. If they think they can start messing around with the scientific descriptor "sex" though, they're living in 1984.
Actually, there are several people with XXY, XYY, XXX or XXYY chromosomes. There are also several conditions where sex can develop differently from what one could expect from XY children.