I think part of it is just comfortability, and ideas one is raised with pacrosby. It wasn't until the last 10 years or so that i started to notice women being overcome with the belief that their boy was going to be be molested or killed if he went in alone. I never saw older boys in the rr at all. Ever. And now it is common place.
I don't care if a male is in the changing room. I don't have anything to hide, and I am old enough to yank a knot in the tail of anyone who chooses to look in. I don't know how I would have felt as a teen girl, because it was never tested. I also don't have girls, and would never think I'd "get" all of the issues in raising them.
My issue with it now (as an old mom of teen boys) is that the act of overly protecting boys in fear of the boogeyman stinks for the boys. And it teaches them that as soon as they hit 13 they transform from babies that need protection into the boogieman, themselves. They become suspect and the object of fear. Really sad, imo.
I've probably wandered off your question too far...sorry!