I'm not sure from a legal standpoint how this would all work out, other than requiring people to have ID on them when using a restroom. For most guys, I think that would be a given (ID is in wallet, in pocket), but for many females, I can understand not wanting to have ID (in purse) with them in the restroom. And for anyone that doesn't have ID on them for whatever reason but really (really) needs to use the restroom, I don't get how we work that out, other than to say go ahead and use the restroom.
But if ID checkers or people that care about checking who's going in are incorrect and exercising authority to restrict use, I just think it is lawsuit (after lawsuit) waiting to happen. And bizarre that we would live in a world where all restrooms now would have people checking.
Would make most sense (from restrictive is righteous perspective) to be able to show genitalia as way to establish gender, but I'm thinking that would be grounds for some other law breaking (indecency) and so is likely off the table. Also, pretty sure most people don't want to expose themselves to another just to gain entry into a restroom. So, even while I do think it would make the most sense, it really is just epitome of the problem in another form.
Sharing bathrooms is the most sensible, but I don't think we are ready for that as a blanket solution that everyone ought to have no problem with. I think it would also make sense to have no gender specific places, but again don't think we are ready for that either.
I think currently the situation is that this restriction business is unlikely to come up often. I highly doubt there will be people checking either as hired staff or concerned citizens in 99% of the existing public restrooms. In the very few cases where that might occur, I'd advise all checkers to be as clear as humanly possible on their reasons for doing so, or risk having a lawsuit brought against them or the people they are representing (i.e. the local, state government). If they engage in harassment type tactics to make the point of restriction, they better hope that person is a dangerous/creepy criminal, otherwise I don't see how that would possibly work out well for them.
I wonder if the kids today will be getting fake ID's just to use the other gender's restroom. LOL.