Sex in the sense that we have been talking about it here is in reference to mammals. The moment you wander outside of the mammalian class of vertebrates these concepts of sex start to become far less applicable.

There are many birds that have more than 2 sexes. Reptiles and invertebrates as well. Asexual reproduction would be classed as it’s own sex apart from any male/female system.

You are vastly underestimating the prevalence of chromosomal variations. They are common, especially among cis women.

I like the way you phrased that at the end. Sexes are categories that relate exclusively to the concept of progeny. If you’re not able to reproduce, you’re already kind of excluded from the sex binary. If we break the human concept of sex down to its constituent parts, it is just “can procreate”. The categories are useful in some contexts, but to state them as universal or to try and extrapolate them so widely is significantly disruptive and unhelpful. Humans are and always have been more than our reproductive anatomy. Your doctor and anyone you want to reproduce with are really the only people who need to know whether you fit into either category.

Yes, there are many species that have more than 2 sexes. Those are decided by scientific consensus.

But sex is ultimately a category to describe the process of reproduction. By definition, this is exclusionary. It’s why conservatives fumble so much when trying to describe sex in terms of actual definitions. Inherently, it is not possible to fit every person into a table of 2 columns in that way. Sex is not a binary because human beings are not binary. There is an incredible amount of variation in our bodies.

Note how they always enshrine gender in biology, but then make all kinds of non-biological statements about what gender is.

“XX is woman”/“Large gametes is woman”/“can conceive is woman”

And then they’ll say

“Women aren’t as aggressive”, “women are more emotional”, “women like being in the home more”, “those are women’s clothes”, etc.

The only reason it’s so important for it to be biological is because of how it punishes gender non-conformity and makes the lives of trans people hell. Like it isn’t ideologically consistent and they know that. They just don’t care. If it was just about genitals or chromosomes, then why is it that gender dictates all these social things about us? The only reason to root gender in how you were born is to ensure gender roles are as rigid and immutable as possible.