I remember taking swimming lessons during winter as a kid at about 9 years old (not sure why winter when I already took them during summer but I guess I just wanted to swim in a heated pool indoors, lol), my mom or dad would drop me off, I'd enter the locker/changing room by myself and obviously undress and get into my swimsuit. This was the early 2000s, so naturally things were normal back then (at least where I lived) and the only other people I'd see in there were other little girls (very rarely some older girls or teens for the older swim groups but they were usually there at different times) or sometimes moms helping the youngest kids get dressed for their swim lessons.

Obviously all of us little girls didn't feel much shame getting undressed, completely stark naked, in front of other girls, but I can't even imagine how scared and nervous we would all be if a boy or a man walked in on us. Even if it was a little toddler boy we would feel it was inappropriate. There are just some spaces that NEED to be separated, period.