Whichever facility they use, they should look and play the part. If the male looks the part of female they should also sit during use so they don't stand out.
My opinion does not include p-k schools. They are well defined. If necessary for schools, a single use could be made available for any student. PE showers can create a whole different response.
Yes genuine trans people will be using the facilities purely because they need them. And that will be the vast majority.But there will be a lesser number who exploit the ability to enter, say, a womens restroom with the view of taking advantage and abusing the women in there. Would it still be possible to do that without allowing self identifying trans people to choose which they use? Of course, but it wouldnt be as easy.
As a father, safety is always on my mind, especially related to children. However, I think it's wrong to criminalize or to focus on labels or identities rather than specific behaviors. Predators come in all guises, and the ones who get away with the most heinous crimes typically slide under the radar under the guise of most respected and least predictable. That's why I follow my instincts and understand that a well-loved "Pastor Bob" is statistically more likely to prey on my sons than a 6'3" trans woman. However, it's even more plausible that both have positive mission, goodwill and love in their hearts, so I'll look for that more than paying attention to their color nail polish.