If a trans woman can't be a biological woman why are they given legal rights?
Originally as a sort of "witness protection scheme". The thought was that some people would be living "undercover" as the opposite sex, after some sort of surgery, as a form of treatment for "gender dysphoria", and that government bodies should let them change some of their paperwork to not "out" them.
It was all predicated on them being undercover, and the assumption that this was necessary for their privacy/safety/etc. None of this would be necessary for anyone obviously trans. Someone visibly male doesn't benefit from fake ID saying they're female.
And a lot of this was tied in with homophobia - in some way a "transwoman" attracted to men is "not really gay", so it's okay. People were prepared to tolerate it in a way that gay people were not tolerated, and the point of being undercover is to avoid homophobia. You still see this in places like Iran, where homosexuality is only tolerated via transition.
Then the GRA2004 came along, prompted by an European human rights case - someone saying the government is recognising me as "a woman", via certain records, but is not letting me marry a man.
The European court (I forget which) said, "no, if you're going to partially treat this man as a woman, you have to do it more fully". So the government introduced the GRA2004 to give more complete recognition, including the ability for a transwoman to marry a man, despite gay marriage still being illegal.
What we're seeing now is this ratcheting further, to say "well, if you're going to let people change sex, why should I have to have gender dysphoria or surgery to get this right?" and "why should there any exemptions - if I'm a woman I need to always be treated as a woman, even in sports, rape crisis centres, prisons"?
And then that's mixed in with the "if I can choose my sex, why can't I choose something other than the 2 sexes?" stuff, together with the use of the word "gender" which either does or doesn't mean sex depending on which argument is being used.
Problem is, there are rational arguments for basing some things on actual sex, and we can make those arguments based on its objective reality, with data and evidence. There's no reason to base anything on a self-declared "gender", because no-one knows what they even mean. Even all the proponents, as evidenced in that Nolan podcast.
You can make an argument that sex doesn't matter, and there shouldn't be any rules about sex. But you can't coherently argue that gender matters more than sex for any sort of public policy.