They frame the argument incorrectly.
It should NOT be based on biological sex at birth..that gives them an argument for discrimination.
Base it on exposure to testosterone.
If an individual has EVER been exposed to significant levels of testosterone they may NOT participate in sports with those who have not been so exposed because it would give them an unfair advantage.
A biological male might never have been exposed to T and a biological female may have been exposed...in such case the biological male could participate in girls sports but the biological female could NOT since she has been so exposed...
Base it on hormone exposure and divorce it from biological sex entirely.
i.e.
If you have been exposed to high levels of T then no girls sports for you.
If you have NOT been exposed to high levels of T then it is fair for you to participate in girls sports.
Regardless of your genetic sex.
Of course, anyone who can make the cut should be allowed to participate in boys sports since that is the top tier...if you can do so without T then you are amazing!
It’s much easier to simply separate athletic competition based on gender. It can be assumed that a biological male will have been exposed to significant levels of testosterone.