>>40821395 (OP)
You are correct in part.
There are, in my opinion, 4 primary indicators of "biological sex", those being (in order of decreasing importance) hormonal, genital, gonadal and chromosonal.
Hormonal sex is mutable and has been since idk the 30s or smth. Plus, it is not necessaily naturally congruent with any of the other three as listed. I consider it the most important as it is the category responsible for sexual function, fat distribution, breast function, to the tinier things like how cilantro tastes and how you perceive spice.
The other sexes are generally only useful for small functions in regards to society. Genital sex is arguably mutable, but also the least strict of each sex category. There are massive variations wrt intersex conditions, which disproves the binary interpretation at birth that most legal systems use for a definition of "immutable birth sex" - an innate falsehood in the context of biology. Whilst SRS obviously exists, in both directions it does not produce an identical product and I would not consider a neovagina female from a material analyst perspective.
Gonadal sex is the definition largely used by anti-trans activists nowadays, as it is the closest they have to an immutable biological binary sex category. However, some people are born without the ability to make gonads and therefore the category is not applicable to all. Whilst TERFs will say that gonadal sex can in those rare cases be inferred, this is by its very nature an extrapolatory and arguably heterogenous definition.
Chromosonal sex is a joke as 1% of the population aren't XX or XY, and therefore the categories must be extrapolated and that is the same problem as with gonadal sex (but to a much greater prevelance).
Therefore, there is no possible manner by which one can establish a binary biological sex category that is simultaneously homogenous and immutable. The best alternative therefore is hormonal sex, it is present in all humans and is easily mutable. 1/2
