I don't think that transvestites should be allowed into the women's bathroom but on the other hand maybe they were more afraid to go into the men's bathroom because they felt they would more than likely be beaten up in there.
It's wrong to attack someone just for the clothes they're wearing no matter how much you don't like it or think it's wrong, but I can see why the men's bathroom would be more intimidating for someone like that and they probably thought the women's bathroom was safer.
I think it's wrong to attack someone like that physically although I think the girls were within their rights to shout at him verbally for invading their space. Women go in there to have some privacy from men when they have to do things like change pads, tampons, etc. and they don't want a man in there accidentally seeing any of that or even a bisexual being attracted to their semi-naked bodies through the gaps in the stalls.