A category separate from 'male' and 'female'. I don't see how anyone can coherently argue against that.
I can see (but not necessarily agree with) the logic that "I identify as a female so I want to compete against other females" but surely no-one can claim discrimination when 'male' means 'biological male, born with a penis and an X chromosome' and if you feel like you don't want to be categorised as 'male' then that's fine. But it doesn't mean you can enter the door marked 'female'. You can compete in the arena where biological sex doesn't matter. 'Male/female/ him/her/they everyone's welcome here'.
Now, how can you argue against that without showing your true hand of wanting to compete against women because you have a biological advantage. Does biology matter or not? Can't be both. Either biology is important, and therefore you must compete in line with your chromosomes or it's not important, how you feel is what's important, in which case, great! You can choose to compete with like-minded individuals.