I think it should be based on appearance. If you look like a man use the mens' room and vice versa. There is not perfect solution here. Our gym also has a family changing/shower area. I suppose that somebody who was having an issue could use this. No matter what we should all try to be nice to each other and calmer voices of reason should try to create solutions that make everybody as comfortable as possible. I'm a chunky middle aged woman. I tend to find a private space to change. I have noticed at our gym that our elderly women are very comfortable walking around naked, putting their feet up on a bench... You could definitely tell if they are male or female if you just didn't try really hard to avert your eyes, which by the way, is the polite thing to do in a gym anyway.