Today I read a Q&A that went something like:
Q. Is it possible for one of my fantasy harpies to be transgender when they're an all-female species? Isn't that like a human who thinks he's an elf???
A. Of course it's possible, because gender isn't the same as physical sex. Write your own fluff to explain if it makes you feel better.
This is going to sound melodramatic, but the answer that my heart was aching to hear was "who are you to tell [PLAYER] what body their character would prefer?"