I cannnot answer this in a short post.
obviously if you claim you are a girl to be able to watch children (or women) in the dressing room, this is a bad thing to do.
but then again, there are people who got confused before and in puperty and now for whatever reason think that they are another gender (or think and hope that life would suck less if they would change gender). some of them do it bc told by masonry.
and yet, I love them all.
and no, in general it is not ok for a man to go use girls bathroom.
but I guess there are some cases where it does not seem much evil intended and does not lead to something bad. (right now thinking of how often had girls go pee at mens bathroom when being at clubs and such bc ladies line so long. obviously not the same thing as a grown man wearing a skirt to go watch children undress. most obviously. but stuff like that cannt be answered in a short post. also, you kind of all know there to draw the line. a line that does not hurt people but protects others.)