Late to the party (sorry!) but wanted to throw my two cents in. I talk about it like a boundary issue. The mean ol' nasty TERFs think that if someone doesn't want to see genitals of the opposite sex in an environment where genitals may be exposed, this is a totally reasonable sexual boundary to have. TRAs say that having this boundary is unacceptable. If someone doesn't want to see the opposite genitals, too bad so sad, because the existence of a single-sex space would be exclusionary.

You could also lead with this: don't they call the women who don't want to pee in a mixed-sex bathroom TERFs? Don't they call TERFs dangerous? So then wouldn't having single-sex and gender-neutral bathrooms work better, since those nasty TERFs would use the single-sex bathroom and leave them alone?

This makes it clear that TRAs are more concerned with stomping over women's sexual boundaries than making sure TiMs and TiFs have a comfortable peeing experience. There are some people who are okay in mixed-sex bathrooms and some who aren't. There is an obvious solution that will make both groups comfortable---offer both single-sex and gender-neutral bathrooms---but that's not the one they're promoting. Instead, they're promoting a system where if a woman is uncomfortable because someone's exposing his penis in the bathroom/locker room, etc., she has no recourse and nowhere to go.

I don't think they've realized either that brushing off assaults in restrooms is basically accusing women of making up stories about sexual assault to get men in trouble.