Let's start with the fact that there are only two options, which in itself is controversial and has supporters on both sides.
I argue that one of the following should apply:
(a) there should be a "Gender" option with free text for selection and not a dropdown menu with predetermined options.
If there are users who fill those gender space, that means it is useful to them, if not to the majority of users but to the few numbers of people using it, we don't need a modification on that part.
The best way will be keeping it optional, let every user decide to use it or not.