Because that’s the argument isn’t it? If trans rights activists and their allies accept that no one changes sex, then what they are really arguing for is that any separation should actually be made on the basis of gender.
As I’m a helpful sort, I’ve started a whole thread for people to make that argument. Off you go fellas. Bring your brightest. Make your case. I really do want to hear it. (And I’m not, for once, being sarcastic).