The two things are not analogous. Thus nobody is swayed by your appeal to analogy.
The two things are analogous in the way that is very important.
A little thought experiment here. Let us say that since female genitals are almost entirely concealed. What's the consequence? Well, it seems that being naked in front of a guy shouldn't bother her, because, after all, her genitals aren't exposed, or at most a tiny bit of them is exposed.
But that's absurd.
These were the two quotes that started things.
Thomas is wandering around the changing rooms with his penis exposed. This is upsetting the girls.
Thomina is wandering around around the changing room with her vagina exposed. This is upsetting the boys.
(Aside: note that "Thomas" in the first quote was a surname, a reference to Lia Thomas, a real person, as are "the girls" in the quote. The second is a hypothetical possible similarity.)
The meaning of the two quotes would be unchanged if the first quote had said "naked", instead of "with his penis exposed". And then the second would say "naked" instead of "with her vagina exposed". The meaning would be unchanged, even if exactly the terms used or the percentage of visible sexual organs exposed may be different between the two cases.
However, what is not analogous is the reaction to the two different situations. The girls would (typically) feel differently about a lone naked male in their midst than the boys would feel about a lone naked female in their midst.
Some have put the different reactions it in terms of danger, and I think that it's important to be careful when using that as the argument. Some people insist that if that is the problem, if we can show that there is not a statistically significant rise in crimes, assaults, or whatever else might be measured, then it's ok to have males walking about naked in the female changing rooms. I disagree. I think the anxiety created by the presence of the naked male is a valid lived experience, i.e. a natural reaction, not created by societal expectations, and is harmful in and of itself, even if the incidence of sexual assault does not increase.