I am clearly failing to get across the point I am trying to make.
Sex is immutable, and it matters. No matter what you wear or how you speak or walk, you will always be the sex your chromosomes dictated when you were conceived. Dress as you please, walk as you please, speak as you please. Nobody should be criticising you for any of that, assuming no taboos on nudity or whatever are being broken. However, if you are male you do whatever you have to do in the male spaces, and vice versa for female.
Wearing clothes some people think of as feminine isn't important at all. It doesn't change your sex. Sex-segregated spaces are important, and there is nothing to prevent you using the one set aside for your space, because nobody is taking a blind bit of notice of the fact that you're wearing a vinyl miniskirt and heels.
I think it's pretty clear where you draw the line Rolfe, I'm just not sure that it will practically work. I think that if men dressing as women, and acting as women, is normalised you make it much, much harder to keep them out of changing rooms. It feels like you are pulling in two directions at once.
I think part of the issue is that the reasons for wanting to normalise men putting on dresses seems to me to be ideological. It's this libertarian, egalitarian idea of people being able to define themselves however they want, and typically being given equal access to things comes into that. You seem to be drawing your line based on some conservative notion of the actual consequences of implementing this libertarian, egalitarian idea of allowing men into women's spaces.
The problem is, I don't think that consequences are really seriously being considered at any point up until the line you personally want to draw. It's not as if concerns about undermining gender roles haven't been articulated on the way to this point, it's just that they were dismissed out of hand in favour of ideology. We all want equality and freedom to live our lives how we want, right?
There is a phrase in Catholic thinking that comes up in Brighton Rock. From the stirrup to the ground. The idea is something like that somebody who has lived a sinful life is thrown from their horse and on the way to the ground regrets it, and is accepted into heaven. One of the messages of Brighton Rock is that it isn't really that easy. You can't live your life under one set of rules and then do a hard 180 just because it's convenient.
It seems to me that your moral reasoning about everything else is based on the liberal equality, liberty etc... principles. I don't think as a matter of pure practicality, one can go with those principles right up until the point you want to defend, and then turn conservative.