Thomina is wandering around around the changing room with her vagina exposed. This is upsetting the boys.
If the reason to not like communal rooms is 'ooh sexual organs' then fair enough. if it's one type of sexual organ you have an issue with then that's your chip on your shoulder.
Let's back that trolley up for a moment.
Some of it is a fairly natural element of modesty around the opposite sex. Humans have been wearing clothes for thousands of years, and nudity is used as a signal of sexual receptivity. So being naked around someone of the opposite sex can very easily be misinterpreted as being open to sexual advances. Modesty regarding nudity is one of the ways that we prevent that.
Additionally, however, there is a pretty material difference in the type of sexual organ. We can play at "true equality" however much we like on line, but in real life, there is a very dramatic difference between males and females. And at the end of the day, there are two things that are rather important.
One is that penises penetrate; vulvas to not. While it is possible for a female to coerce a male into sex, it is incredibly difficult for a female to forcibly rape a male. The same isn't true in the opposite direction.
Second is that there's a difference in physical strength as well as aggression. There's simply no way around the fact that males commit over 90% of all sexual crimes, close to 90% of all violent crimes, as well as the overwhelming majority of domestic violence and child sex abuse. Not all males, certainly... but so massively skewed to male perpetrators that it's a material concern for females - who are disproportionately the victims of male sexual crimes and domestic violence.
It's naïve to think that the risk and the dynamic is identical between males and females. You stick one male into a female changing space, and the male represents a danger and a risk to all of the females in that space. You stick one female into a male changing space, and the males all represent a danger and a risk to that single female.
So this whole line of "oh you're bigoted against genitals, that's YOUR problem" is baloney.