From my understanding of the Supreme Court ruling, sex has to mean biological sex in the act because that’s the main reason why women are discriminated against, so that is the characteristic that has to be protected. If trans women are included as women then biological sex is no longer the protected characteristic but gender identity - and employers could go back to asking women if they intend to get pregnant in job interviews - as they used to before the Sex Discrimination Act.
This seems to me to be a pretty good argument for the necessity of sex meaning biological sex but it doesn’t seem to be used. The focus is much more on single sex spaces. Is there a reason for this?