This prophylactic argument might be a reasonable one if there were an epidemic of transgender women (or, for that matter, cisgender men) harassing cisgender women in the bathroom. But, instead, it is virtually unheard of. The empirical evidence is that, today, the combination of informal social norms and retrospective laws against, e.g., lewd conduct and sexual assault are more than enough to discipline' people's bathroom behavior.

we should legislate that all men must be under constant video surveillance. not being on camera makes it too easy to break the law.