@NecessaryScene1
You see if humans
could change sex, then it would be clear what we as "sex matters"/GC people are worrying about is
current sex,
not original sex.
(Although I could imagine there could be some difference between "born as male" and "converted to male later" humans in this hypothetical universe, the current sex would be a big deal.)
Actually I don't agree. I think the different socialisation that male and female people are subjected to from childhood and throughout their lives - everything from the characters, storylines and "normal" they grow up with in advertising, books and films, to what they see in adult interactions and behaviour, to the (often subconscious) way their caregivers treat them, to expectations from other children which are formed by a myriad of variations of the same social pressures - give hugely different outcomes in how they see themselves, and what behaviour and abilities they and others expect from them.
Even if trans people in some as yet undetected yet deeply real way empirically are the gender they identify as just as much as a "cis" member of the same gender, they still have a totally different lived gender-experience. And that applies to their pre transition life as the opposite gender, but also in their post-transition life because they are a person who has actively chosen their social gender constraints rather than having them imposed from birth.
So if actual sex change were possible, current sex would be the criteria for many of the reasons we need single sex provision, but birth sex would still be important to things like political representation, career and education opportunities and so on.
And that's why today, when actual sex change is not possible but there is an ideology agitating to replace all use of sex as a social or legal descriptor with a self-defined gender identity, it's important to accept that single sex provisions are valid and necessary and it does not invalidate a trans person's gender to sometimes be excluded based on their sex.