Referring to "biological sex" in the context of trans women is almost exclusively used to misgender us and deny not only our everyday lived reality, which is clearly that we are treated with all the contempt and objectification patriarchal society has for all women and then some, but it also betrays a superficial, actively anti-scientific understanding of what constitutes biological sex in the first place.

Biological sex isn't a discreet, unified characteristic, it's a composite of different biological features that then get conveniently subsumed under one label.

Nowadays you will frequently hear that biological sex equals chromosomes, but i do not know a single cis person who actually knows their karyotype - in fact, a not insignificant number of them have a karyotype that does not fit what you'd expect for their phenotype (see Klinefelter, Chapelle and other variations of chromosomal sex). And when you look at how transphobes want to police access to single sex spaces in a way that specifically excludes trans women, chromosomes are only ever used as a bad faith argument and never a foundation for exclusion in practice.

What transphobes instead obsess over are genitals. Now, if you have functioning gender affirming care (which obviously does not apply to any place on earth, particularly not the UK), a majority of trans women would in fact have a vagina. In my social circle, which consits almost entirely of trans women, we're slowly reaching the point where a majority of trans women do, in fact have a vagina that is not distinguishable from that of a cis woman by any means. I won't even get into the point that a woman's penis is functionally not the same as a man's for a variety of reasons, it will likely break your brain if i do. But when you press transphobes on this issue, when you ask them if they actually want genital inspections and if they would then sperate between people with vaginas and people with penises (what to do about people with both is a question that never occurs to them, even though such people do exist), they quickly reveal that they do not mean that, either. And they do not mean secondary sexual characteristics like facial hair, breasts, tone of voice etc., all of which are mutable and not equally pronounced among all men or all women.

No, what transphobes actually mean by biological sex is socially enforced sex. It's always nothing but that. I could go on, this issue is a lot more complex and would require that i throw a lot more commonly held assumptions about biological sex out of the window, but let's leave it at the fact that these discussions and these laws and these means of exterminating us are never actually about biology, as our biology is usually a core part of what we wish to, and usually do, change during our transition.