@Vgal #115339 and Skide
And where not available?
Well, the women's restroom. It would be dangerous for a transwoman to use the men's restroom. She would face, in the very least, nonverbal signs of disapproval or some harassment and it only gets worse from there. I absolutely don't expect transwomen to use the men's restrooms for that very reason.
That raises the good question of why transwomen shouldn't just use the restroom congruent with their gender all the time: they should.
You and Skide both make supremely valid points against which I don't know how a TERF could argue in good faith. I can't argue against them in good faith.
Having known a few transwomen, I have a slightly better idea than some people of how expensive it is, and how time-consuming, to get the gender affirmation surgeries that would complete the process of transformation.
Even knowing that, however, I didn't realize getting help was *that* hard. I knew it was hard in some places in Europe, but five years in the UK? Meanwhile, the person waiting is suffering with gender dysphoria.
And truly, it's not like we can look down people's pants or up their skirts to determine if they are "real" women.