It would be illegal to turn away a person undergoing gender reassignment or intending to do so under the equality act, unless they risk assessed each person entering the changing room against whether stopping them would be proportionate to a legitimate aim.
Again, is it completely beyond you, to drag your eyes off the men, for just one second? Even a nanosecond?
We can exclude all the men, because a legitimate aim is that the women don't want them there. And has fuck all to do with the intention of the man involved. He doesn't need to be considered.
And secondly, even you are never going to be able to come up with a scenario where a primark changing room assistant can not only risk assess a man on the spot, but get away with claiming they constitute an unacceptable risk on the basis of their personal two second risk assessment!
Unless, of course, like some of the Twitter pics, they are wielding an axe, or a sword.
Crikey, if Primark changing room assistants can risk assess the male population to that degree, I can guarantee you, they're not being paid enough.