Why though? This is exactly what we are debating but you are just repeating it.
Why can’t one say:
ok I’m discriminating against men on the basis of birth sex, for x, y, z reason (legitimate aim).
However because x, y, z reason (legitimate aim) does not in fact apply to trans women , because (eg reasoning offered by court elsewhere as to impact of gender reassignment) , and because the impact on trans women (because a,b,c) would be much greater- for trans women this sex discrimination is a) not a legitimate aim and b) disproportionate , therefore unlawful, and also being not legitimate and disproportionate in its application to trans women because of their gender reassignment , indirectly discriminates against them due to the protected characteristic of gender reassignment. So therefore unlawful.
But WHY, but WHY, but WHHHYYY can't men be women? Good grief, you sound like a toddler.
Because X, Y, Z legitimate reason does apply to trans women the same way it applies to other men.
Because there is absolutely no evidence that trans women pose any less risk to women than other men do. (But there is quite a lot of evidence suggesting the opposite.)
Because there is no demonstrable impact on trans women of being excluded from female spaces, except that it doesn't affirm their private, personal identities, which isn't a valid reason.
Because even if there were a demonstrable impact, it wouldn't be relevant because they are male, the space isn't for them and so the impact of excluding them from it doesn't need to be taken into account.
Because the space is for female people and so the only relevant question is what impact it would have on those female people if male people were to be included, and whether the space would still serve its intended purpose as a single sex space from the point of view of those female people, which it wouldn't.
@Tandora, you really have to get your head around the fact that the Supreme Court judgment marks the end of absolutely-fucking-everything being considered through the lens of what male people (including trans women) want, and the beginning of the next wave of actual feminism, where society begins to reawaken to the fact that female people are actually human beings in our own right and do not exist merely to serve and validate male people.