Look, law or no law, pinning your safety on a sign dictating which set of plumbing is allowed in the bathroom with you is as stupid as assuming that a piece of paper will keep your abusive boyfriend away. If creepy rapist kidnapper dude decides that he’s going to hunt women in public bathrooms the fact that his plumbing doesn’t match the sign on the door isn’t going to stop him. MAYBE allowing folks with different plumbing into a marked bathroom will up the risk level. But I have my doubts. Creepy rapist kidnappers are not restricted to the opposite gender, or to molesting the opposite gender. Public bathrooms are simply not, and never have been, a place to lose yourself in the white zone.
Honestly, the simplest, if costly, solution is to turn all bathrooms unisex. Change the partitions to full length, and get rid of the urinals (seriously, very few guys have them at home, so why do they need them in public?). That way parents can stand within reach of their child of opposite sex while the child uses the bathroom. Guardians of the handicapped don’t have to find a “family” bathroom in order to help their charges on the toilet. And no one flips out at the apparent gender of the person who just walked through the door.
The whole thing could even be phased into usage so that the financial burden isn’t as painful. Swap out the partitions for full length, partition off the urinals with signs on the doors “URINALS ONLY”, and change the signs on the main doors. Then later, as regular maintenance has to be done, swap the urinals out for toilets.
It’d even save money in the long run for new construction as they’d only have to put in one, slightly larger, bathroom in new buildings instead of the two divided bathrooms.
Frankly this sort of thing should have been happening a while ago. And not because of the transgender issue. There are plenty of people out there who mentally match their plumbing, but who’s looks do NOT match their plumbing, and who then get hassled for using the correct bathroom. Plus see above comments about other reasons why someone of the opposite sex might need to be present near by.
And yes, the same sort of thing could be put into place in locker rooms and the like. Though the solution for how to handle the transgendered folks who wish to play sports and whether to put them on the men’s or women’s teams would have to be thought through a bit.