Why would it be unconstitutional? If you have even the slightest notion that it might be you should probably go and read the constitution and point to where it mentions where people can piss and shit.
It's a question of public paranoia, no more, no less. There are perverts around every corner. Men don't respect women and will run around in dresses just so they can get a look at women pissing. Come on people. It's a bathroom. The majority of people don't find the opposite sex on a toilet sexy, and the people who do get off on that kind of thing have probably already committed crimes if they're that desperate to get their rocks off because they have mental issues far beyond just wanting to wear clothes of the opposite sex.