Generally speaking, public restrooms aren't court rooms. And property owners have multiple interests in ensuring that the correct people are entering the correct facilities before a problem arises rather than waiting for someone to act "guilty".
Within reasonable suspicion. I can't think of a restaurant that checks customers licenses before allowing them to enter a restroom, so clearly establishments don't see enough cause to parse their poopers preemptively.
A trans-looking individual is not reasonable suspicion. The reasonable person standard is widely accepted in law, and so I don't see why it can't be used as the standard here.
That's my take on this as well. I honestly don't see it being a huge issue, particularly once social expectations have shifted to actually include trans peoples. Right now, social pressure is against them both ways--if a trans woman is trying to live as a woman, men don't want her in their restroom, and if a trans man is trying to live as a man, women don't want her in their restroom. This is, of course, assuming care about the gender barriers that are in place.
Honestly, I just don't see why it would be an issue. I get why a bar or restaurant owner would prefer a system that stops problems before they begin. I also don't think our system does that now. Nor is there any way I can think of that stops potential issues, before we even add trans peoples back into the mix.
It almost seems far better, to me, to have a socially agreed upon standard for using restrooms. I can't help but feel that the end result would be a net drop in issues. And I can absolutely be wrong here--this is a gut feeling. I have no objective evidence one way or the other. But if trans people aren't welcome in either bathroom right now, in theory it can only get better if there's a system in place that establishes their welcome to one or the other. That's my take on it at least.
There are always going to be creeps doing their creeper things. You deal with them as you've always dealt with them. If someone has a better idea on how to stop that, I'm all ears. I don't. And I don't think using something that is already an issue actually helps.
And Joph, you're right--I should have been a lot more clear in what I meant by everyone go where they want. I should have said everyone should go to where they feel most comfortable, and from there you rely on social boundaries to protect the people inside. I have difficulty imaging that a significant population of new deviants is going to emerge by allowing for overlap in the gender domains that bathrooms serve. That's all I meant by the peepers gonna peep comment. If they'd be willing to peep in this new context, I seriously doubt they're somehow less willing to peep now. To the best of my knowledge, peeping behaviors primarily stem from social issues and anxieties when dealing with the opposite gender. You probably aren't going to just brazenly walk into a domain specified for the opposite gender, you're going to sneak in.