edit: I am a man and the only man in this scenario
This happened some time ago. I was driving and stopped on a gas station. probably for coffee and had to go pee. I saw a long queue of around fifteen to twenty exclusively 30-something girls, definitely bit older than me. They probably were a sports team, since there was a hired couch bus waiting outside and they were mostly wearing sweats. There were separate stalls for men and women (one each), so I went for the men's room. It turned out locked, so I stood just outside it. One of the girls in the queue said that the back of the queue is "back there". I replied "sure, but I'm going to the men's room", understandably assuming they were queueing for the ladies room. To which she said "yeah but there's one queue for both". I am familiar with the concept of shared queues, but mostly from supermarkets or post office, where you would queue for several checkouts and just go to the first one that is free. Never encountered shared queues for gender-separated toilets, so I said "but the toilets are separate, I'm going to the men's room and you can queue for the ladies room" and simply went in without any more protest from them when the men's room emptied (and it was another girl in there).
Were I the asshole?
Uh.. Yeah.. The door opened and it was another girl in there proving they were in fact all in line for both. You cut in line.
In this case, if he did get in line and when he was in front of the line, would it be acceptable for him to use the women's stall if the men's was occupied? I suspect that would not go without objection, but I could be assuming wrong. That would be pretty unfair if they were using the men's.