There are tricks to hide one's genitals even when disrobing in the view of others cannot be avoided. Transgender individuals who wouldn't raise an eyebrow when entering, very likely have mastered that art. Which is why the suggestion by some that transmen should use the women's changing room is so unhelpful; no one will ever notice that they are not (biological) men.
This is true. And I think that many of the "anti-trans" side of this debate have little issue with trans-women who try to be discrete. (By discrete, I mean not standing around letting it all hang out...turn back, pants down, pants up, done.)
However, there are those who say that trans-people should not be expected to hide their bodies or practice discretion when in locker rooms. (I don't believe that this is a universal within the trans-community.)
But what is good behavior in a locker room? Informally, discretion probably counts for a lot. But if you make a law granting access, how do you mandate good behavior in a non-subjective manner? It's easier to tolerate (with "good" behavior) informally than it is to legislate something with definitions of bad behavior.
I'm not in favor of bathroom laws in either direction, because legal terms don't allow for situational flexibility. And the problem, in my view, is not trans-women, it's cis-men who are creepers that may exploit loopholes. If segregated spaces are based on gender identity (as opposed to sex) and gender identity is a matter of self-determination without any kind of verification or certification, then it is impossible to tell
anyone that they are in the wrong locker room, let alone enforce it. Effectively, you have gender-neutral bathrooms. (Well, not quite, as
most men will use the men's room and few women will use the men's. But it's an opening for those who want to cause trouble.)
A couple anecdotes regarding unisex bathrooms....
I have a friend who works for a Big Ten University in another state. (I'm not going to name it because it's not my story.) Last week he mentioned that the bathrooms in his building were going unisex. He was surprised how much push-back there has been from the females in his building, most of whom are pretty much as liberal as you can get.
In many places, single stall bathrooms have been changed to gender neutral. Which only makes sense, really. However, I know one place where the women's room was made unisex. The men's room, however, had a toilet and a urinal. Since there was a divider between, it was considered two-stall. So that's still the men's room. There is now a men's room and a unisex bathroom. There is no women's room.