""If we were in a world where all that was being argued for was, "some men want to present as a woman might socially be expected to, but obviously he's not actually a woman and we wouldn't dream of pretending he is in matters of law or anywhere it might have a detrimental consequence [to use your phrasing], such as safety," I think you'd be hard pressed to find anyone on this board having a problem with that and we would categorically not be in the position we are in legally and, say, in sport."
The differences are more about what qualifies as a detrimental outcome not to mention the dehumanising & demonising framing."
Honestly, this is excellent if we have in fact reached agreement that in reality of course men are not women, however much they may wish to present like them (and even if on occassion they succeed in fooling people into thinking they are the other sex in social situations).
If you are with me that far, then we are a lot further down the road of productive discussion than I imagined, and all we need argue about is in which situations sex matters.
Interestingly, that leads us precisely to the FWS Supreme Court judgment (which is cogent) where the answer is, essentially: You must have mixed sex everything unless you can show that having separate sex provision is a proportionate means of achieving a legitimate aim. If you successfully show that separate sex provision is a pmoaala (for which you would have to establish based on data and other evidence, that not to do so would negatively impact either men or women as a sex class), that is the end of the matter. There is no question that men with the other characteristic of a particular gender identity can be included in the provision for women, any more than you could include any other sub-set of men. If you say that, actually, you can include some men in the women's provision with no problem or detrimeny then your pmoaala basis for a single sex space is fatally undermined and you clearly don't actually need a single sex space. Therefore the provision must be entirely mixed.
That is the legal argument re the EA2010 but also simply the logical one. If I have data that collected the incidents of assault against women in single sex changing rooms, prisons etc as compared to mixed sex ones and use that as the basis for my pmoaala, there is no basis whatsoever to make an exception for the section of the opposite sex who think or present in a traditionally feminine way, any more than it makes sense to make exception for any other sub-class of men. Because that's not level or the basis on which the separation was justified - it is entirely outwith relevance to the reasoning.
Essentially gender (or what you seem to be calling 'social' sex) is something completely separate from the question of in which situations sex matters. It doesn't even come into the discussion because we both acknowledge that sex is, in reality, biologically determined and is not affected by any other characteristic a person may possess.
We could go into chapter and verse about the facts and data of what causes harm when men (any men) are included and where women need single sex provision, but we have probably taken up enough of this thread and we would happily be back to arguing on the basis of factual evidence.
I am DELIGHTED with our progress.