You're using the word sex. There is a known, massively complex, relationship between sex and gender but it's not 1:1. For anyone. Or, if you think that's not true, then please describe to me how they're identical. In a group of men, in any place in the world, you'll find wildly varying accounts of what the male gender "is" all the way down to how their bodies should look.
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.
> There is a known, massively complex, relationship between sex and gender but it's not 1:1
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).
I think it's possible to have a simpler social classification system without treating trans people poorly. There are many other rare mental issues that we can't treat well and that we don't bend all of society to try to alleviate for the victims. That doesn't mean we have to hate on the victims or treat them poorly.
There are a a few pretty big upsides I can think of to using a simpler social gender classification system: less social friction/discord/controversy (too much of this and your country falls apart and everyone is worse off), less social confusion/cognitive burden (pronouns, as one example, have basically become a third name that you have to memorize in addition to first and and secondary names whereas they used to just be a derived property that you didn't have to memorize).
Simpler than "treat people how they look and act, update your assumptions if corrected"?
This system can break down for some trans people (although I bet most make excessive effort to present as their preferred gender), but it also breaks down for effeminate males and masculine women (and I bet this is the larger category). Despite these difficulties, it's worked for literally all of history until people started pretending they don't know how pronouns work. Test it yourself by taking a look at Lynn's picture and asking which pronouns you'd use.
I'm not sure how invoking genetics simplifies anything. Have you had your sex chromosomes checked? I haven't, and triple-X and KS are often undiagnosed! I'd hate to have to update my drivers license to "unclassified" in my 30s.
Yes, it's simpler because it's immutable. It's the mutability of identity that causes so much confusion and complexity. In Lynn's case she was a man the first 30+ years of life and even married and had kids. Then later switched genders, names, and pronouns. That's anything but simple since now every person and computer database that ever knew her now has to track and reconcile 2 identities. It's incredibly confusing when people change their identities later in life, especially if you are out of the loop. Even more confusing if people change identities multiple times.
I actually don't really care too much which classification system is used (chromosomes vs. observable genitalia at birth vs. something else), I just think the pros of collectively treating certain aspects of identity as immutable such as gender, race, and possibly even name, outweigh the cons of making a tiny fraction of the populace slightly more uncomfortable.
You're arguing that somebody who wears dresses and has breasts should be referred to as male? That pronouns shouldn't be based on appearance or preference like we've done throughout history, we should instead invent new classifications based upon unobservable characteristics like genetics or genitalia at birth? And that this is all being done to reduce confusion?
I think we're just going to have to agree to disagree. I personally think it's simpler to continue to refer to "Chelsea Manning" as "Bradley Manning" with he/him, even if they decided later to completely change their identity. Close friends can call them "Chelsea" out of respect, but society should not rewrite history to erase all traces of "Bradley" in order to accommodate one individual's mental health struggles. I find (the history rewriting) to be extremely confusing and, frankly, Orwellian. That said, if I'm interacting with a trans person that prefers some name/pronouns, I will use them out of respect, even if I fundamentally disagree with a lot of trans activism.
Do you hold the same views about marriage? That changing one's last name is is confusing and Orwellian? That close friends might humor the individual out of respect, but society at large should refuse to allow history to be rewritten?
I personally am against changing of last names at marriage and encouraged my wife not to change hers.
But to directly address your point... the main difference here is that "dead names" and "dead pronouns" are considered harmful and/or offensive and are therefore actively hunted and scrubbed from history by activists, unlike maiden names which are considered benign.
The classification you've described is not valid for use with Homo sapiens, neither today nor in the future. It's based on the assumption that our genes control our organs, when they're merely a weighted suggestion at best. And, the Y chromosome is gradually withering away in humankind and is expected to disappear someday. Human beings can develop fertile male reproductive organs without it (see below), and evolution abhors exceptions that have no benefit and many drawbacks (such as colorblindness).
> 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.
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.