They can't do it in their hands, so which bathroom are they supposed to use?
In the women's bathroom they may be ejected by worried females because for all intents they could be a student on rag week, guy dressed up collecting for children in need, on a stag night or a pervert trying his luck and those are just a couple of options.
In the men's bathroom they may feel uncomfortable, be ridiculed.
It's not so clear cut, if I were waiting for one of my daughters and she came storming out of the bathroom saying "dad there's a man dressed as a woman in there" I'd be liable to at least ask questions, it's my job as a protective father. Some hot tempered types may forgo the questions and proceed with a Glasgow kiss, it probably has happened. I'm very reasonable and liberal in my views some are not. It's a mine field out there and asking people to simply presume that any person who may be a male dressed as a female in a woman's bathroom is a harmless transsexual just won't work.