Fascinatingly frank admissions in this transphobic brief to the Supreme Court. They admit that "sex" is not sustainable as a reductionist scientific concept!

Because there's just too much causal messiness (in genes, expression, hormones, etc) under the rule-of-thumb. https://www.supremecourt.gov/DocketPDF/24/24-43/374986/20250918112750820_24-38%20and%2024-43%20Amici%20Brief.pdf

Amazingly, the brief centers its argument around irreductive conceptual classifications being higher in "naturalness" than reductive ones.

Now, of course, irreductive notions of "sex" can be more pragmatically and perspectivally *useful* to humans, BUT USEFULNESS IS NOT NATURALNESS!!

The actual structures of underlying nature are infamously not aligned with human intuitions, cognition, or usefulness in everyday life. Indeed one of the ways we can tell quantum mechanics is REAL, is how alien it gets.

By contrast, "sex" is, of course, not real.

Sex is a cluster concept, a crudely aggregate "rule of thumb" that humans constructed to smush together countless dynamics for a variety of reasons.

Pragmatics and political perspectives are baked in. And such are the OPPOSITE of naturalness.

And the brief is very explicitly attempting to leverage 'naturalness' in ways that appeal to facts of hard sciences like physics and chemistry, because they compare trans people to "fool's gold" -- a situation where "gold" can have a far more exactly reductive definition.

Note the contradiction: the brief admits that "sex" defies a validly reductionist definition, but then appeals to an intuition pump example wherein the sharp distinction between gold and iron pyrite IS validly reductionist.

Gold is a natural category, "sex" is not!

Now the reason that the brief doesn't want to allow for reductionism is because reductionist analyses of "sex" immediately point out the tangled arbitrariness of genes and hormones. Unlike the 79 protons specific to Au atoms, there is nothing firm underneath on which to prop "sex."
And of course, while the authors of the brief are big mad at self-identification not being very "natural" as a category, they know that that chromosomes and genetics are invasive to measure and messy to categorize and really really do not want to accept hormone regimes as a base.
Now I'm not a partisan of hormone regimes somehow being the one true underlying determinant of "sex" -- sex/gender is irreducible and nonnatural because it is socially constructed for human ends -- but it's worth noting how absurd the brief's anti-reductionist "naturalness" is:

The brief is even explicit about the intersection of this absurd notion of "naturalness" with the innate crudeness of state violence.

It literally argues that because laws make arbitrary cleavings on things like age (denying rights to certain young individuals), the same should be done with sex.

This argument is essentially that because judges and lawmakers make sloppy and crude demarcations, that crudeness should be rewarded and enshrined as intuitive awareness of some divinely natural categorization, never mind the exceptions and lack of reductionist grounds.
Now, as an aside, I don't personally care about the continuance of segregated sports. We should be rewriting our genes, taking chemical enhancers and getting cyborg enhancements in ways that erode any lingering premise of "natural talent" in sports, as well as any "sex" distinction.

@rechelon For sport as a participatory activity, the obvious thing would be to divide people into groups of roughly equal abilities, without bothering to classify by sex or age. Sports already have separate leagues after all.

Sport as a spectacle is a different thing, now turned into a big business based on mass spectatorship. The women's events are segregated and give women a chance to win the big prestige and prize money. This could die for all I care, so I don't really advise how to fix it.

@rechelon I'm happy to leave it to sports people themselves, if they want to have separate events for women, to provide their definition of "women". They have been doing this for years anyway. I don't see why politicians or random pundits should be involved.
@ghouston @rechelon just have hormone classes like you have weight classes imo