I've used the men's bathrooms since I was 15, pre-T, pre-coming out, sometimes even not-passing, and no one has ever said anything or given me any weird looks. I live in Europe though, so maybe things are different here than in the US, who knows. Also, when all the stalls are filled I just stand there waiting, what else is there to do anyway? It's not like you have to wait for long, it's usually less than a minute! And if you get bored waiting you can always check your phone, wash your hands, look in the mirror etc.