The one according to their plumbing. This way they are confronted with their illness, and everyone else knows who to shun.
Who does the plumbing check? Will there be genital inspectors standing by? Because if I saw the "dude" in the photo I posted using the men's room, I wouldn't give it a second thought. If I saw her complying with the law and following my wife or daughter into the female's restroom, I'd try to put a stop to it --and then I'd be the one looking at prison.
What about gay dudes? What restroom should they use?