Because when I was traveling with my ten year-old daughter, she was terrified by a man in a dress than came into the women's bathroom while she was using it in a restaurant where we'd stopped to eat.  She had the right to feel safe from male intrusion while she was using the toilet.  She's grown, has three kids of her own, and still doesn't want males coming into public restrooms while she's using 'em.  Privacy at times of vulnerability is a natural right.  It used to be a legal right.  Show me a female that DOESN'T feel somewhat vulnerable while taking a dump outside her home.  The presence of male strangers when females are in vulnerable positions is probably contrary to our wiring, as for most of our history such situations frequently ended badly from the females' standpoint.  "Accommodation" should be give-and-take.  If you insist I accept males getting "married", my daughter should be able to pee in privacy and not in the presence of a male stranger (cross-dresser or otherwise).