This has been explained in multiple different ways.
Unfortunately the poster has ignored the issues around this. They erroneously cannot comprehend that a male person is the one who needs to be removed from a female single sex space. Partly because, from the language they object to, they don’t seem to think male people are male people at some special point.
They cannot see that if the third space is open to all, then others of either sex will use it and no one is passing any kind of judgement. Whereas, by removing female people from their rightful space it is making a morality judgement on those moved.
It is not that hard to comprehend. Unless maybe you won’t read it, or choose to ignore it or choose to not understand it.
It was never an equitable solution to move women and girls who object out of their space.