First of all, it’s highly unlikely that a trans woman who hasn’t had surgery would be comfortable showering openly in a locker room, anyway. Most trans people want to blend in when they use restrooms or locker rooms. Secondly, once trans people do begin to transition, using the bathroom or locker room associated with their assigned gender can be extremely dangerous and uncomfortable for them. A trans woman who presents as a woman will probably stand out much more in a men’s bathroom than a woman’s bathroom. People usually don’t see your genitals when you use a restroom (at least not up close), but they do notice if you look like a man or a woman. Finally, I think the idea that men will pretend to be trans women in order to harass women is a bit of a strawman argument. First of all, you can’t guarantee that anyone in a restroom or locker room has innocent motives. If someone is behaving inappropriately, then kick them out or prosecute them for that. And if someone isn’t making any effort whatsoever to present as a woman, but insists on using the women’s restroom, then yes, that’s going to look suspicious. Considering the discrimination trans people, especially trans women, face, I really doubt that many male predators would be willing to live as trans women just to have access to women’s bathrooms.