Nudity is hardly a problem, since the issue is men in women's bathrooms, and women's bathrooms have stalls with closable and lockable doors.
You might get a glimpse of someone's pants around their ankles.'
The big issue involved in all of this is the fact that children are often bullies,and no one gets bullied more than the effeminate boys and I imagine that would include transsexuals as well. In my high school and junior high and the ones I taught in, locker rooms were where all the really serious bullying took place. Because the issue here is not protecting women from that rarest of beings, the crossdressing male rapist, but preventing children from not being bullied.