Question for any lawyers reading

As I understand it, other protected wotsits are based on you discriminating against someone based on either their sex/sexual orientation etc of your perception of that. So, for something to be homophobic, you don't have to first prove that the supposed victim is gay. Whereas as far as TRAs would have it, if John from accounts turns up in a dress and now wants you to call him Samantha, and treat him like one of the girls -- you would be required to suddenly start treating him differently. How does my behaviour towards someone be okay one day and not the next based on what has changed in between their own perception of themselves and their supposed disclosure of that fact to me. And even more confusing if John didn't dress differently, or name change, or shave off his beard, just announced that he is now a woman...............If your behaviour changes because of that declaration, isn't that actually sexism (if you believe that he is a woman). If someone declared any other personal belief (like they'd become a muslim) and you then started treating them differently because of that, then that would problematic, surely?
Gender presentation, I can see how you can have a meaningful anti-discrimination law based on that, but self-declared identity? Always ends up descending into sexism, frankly.