I've seen this point raised in passing across a few threads before but want to give it its own.
Let's take the TRA position. Biology and biological sex are irrelevant, or at the very least unimportant. What matters is someone's gender identity. That is what makes them a man or a woman. This is also separate from their 'gender expression' so you can wear skirts and heels and identify as a man. Or be Danielle Muscato and identify as a woman. It's about an internal sense of self.
Accepting those hypothetical premises, what is the reason for dividing changing rooms between those who have a 'man' inner essence and those who have a 'woman' one? Why is it necessary?
When teenagers go camping, what reason is there to divide the tents between those who experience a 'girl' gender identity and those who experience a 'boy' one? What about boarding school dormitories - why is it important for young people to sleep in a room with people who have the same internal sense of identity as them, and wrong to share with those who have the opposite identity?
Why should sports competitions be split up based on whether people identify as men or women? Remember, it is identity that matters, not body parts, hormones or presentation. Why shouldn't everyone compete together? As a bonus, that means that those who identify as something other than 'man' or 'woman' can also join in!
I know (and share) the GC position on this. I'm honestly curious about what the rationale is on the other side. Is there one?