I am really confused. If it's not to do with GRCs but absolute biology then where are trans men to go to the toilet? I don't think you think they should have to go to (biological ) men's toilets since biology trumps all in your argument. I know you will say third spaces but, assuming they aren't everywhere, where?
I know this is sometimes used as a gotcha but , in my case, it's a genuine puzzlement. If the person I know who was born biologically female entered a female toilet you would definitely perceive there was a man in the loo.
Sorry, I know you have bowed out life so I am wondering perhaps into the ether.