And, gender aside, the most important thing to consider is the existence of intersex people, the diaspora of their bodies, and to consider how you think we should talk about them. Sex is, even outside gender, in fact not immutable. It's biology. These people are also historically denigrated.
Please imagine how ~150 years ago we collectively thought (and some people still think) that a person's race determined their intelligence. Historically "fundamental truths" usually end up with people being thought of as subhuman. The "fundamental truth" of sex, as it's presented by those who consider trans folks not people, is the same sort of truth. Biological sex is a spectrum, demonstrably. Gender is also demonstrably a spectrum. I don't care what people believe sex is or isn't. I care that we treat everyone, no matter how "weird" with respect.
You could argue though that it's only complex because we (collectively) decided to complicate it in order to accommodate the preferences of small subsets of society. In some other country, society might collectively decide not to accommodate said wishes and instead treat sex and gender as immutable and indistinguishable. Such a course would result in a simpler classification system: XX = female, XY = male, anything else = unclassified (genetic defect handled on a case-by-case basis).
> a simpler classification system: XX = female, XY = male
This would, as with all other such systems that refer to X/Y chromosomes, be invalidated immediately upon contact with reality. I estimate that a couple million people worldwide have one set of fertile reproductive organs that do not match the binary view described – that is: men without, and women with, a Y chromosome.
> it's only complex because we (collectively) decided to complicate it in order to accommodate the preferences of small subsets of society
None of these people selected a "preference" at birth, and may go their entire lives and have children without ever realizing that their chromosomes and their reproductive organs do not match the XX/XY binary you've presented.
> anything else = unclassified (genetic defect handled on a case-by-case basis)
This would mislabel XXX, XXY, XYY people as "defects" for genetic circumstances that do not necessarily have any visible presentation, that people may not be aware of at all.
It also mislabel some, but not all, intersex people as "defects". Intersex people span the entire spectrum of known chromosome combinations in human beings: Human bodies produce one or more sets of (often) fertile reproductive equipment regardless of what chromosomes are or are not present.
Ironically, then, focusing on XX/XY classifications while disregarding the realities of human biology always results in an invalid classification system that is more likely to harm cis people than intersex people.
Evidence and data seems to support this as we learn and discover more.
