Happily married 50 year old men are just as likely to make teenage girls uncomfortable by being in their changing rooms as non happily married ones. It's not just about safety.
The rules are wrong. It's not acceptable for an adult male to change with and swim in competitions for girls and young women, whether they are a pervert or not.
No, never. There was one week at our leisure centre when they made the Women's unisex because they were refurbishing the Men's. It was very unpopular, especially with parents who were taking kids for lessons. Fortunately lots of men chose not to do their usual swimming that week, because good men stay out so bad men stand out.
You can be someone who doesn't fit into traditional gender roles (most people don't) and still respect that there are single sex spaces for good reasons. Melody Wiseheart could be gender non conforming in the Men's changing rooms surely.
Out of interest, are all the changing rooms mixed sex? If there are Men's, Women's, and Mixed, then those using the mixed have consented to it. If there is no choice, they haven't consented and you can't really know if they are happy or unhappy about it.