Being transsex is a physical disability
I have never understood why the predominant understanding (by those who even acknowledge it's medical) is that transsexuality is a mental condition.
Any given cis man can have any number of developmental disorders, or just be statistically unlucky, and have one or more of the traits that trans men typically have. E.g., he could be 5'2", he could have hormone issues and need TRT, he could be infertile, he could have wide hips, he could have gyno, he could have a micropenis or some other genital disorder/intersex condition, etc. Of course, having multiple of those things in one person becomes more unlikely the more of those things you add, because they're typically independent events so the probability goes down for each new criterion you add.
However, sometimes a man can have all of those things at once. It's still rare, but it's much more common than it would be to have all of those things independently coincide, because they have a common cause. When this happens, we describe the person as "trans."
Due to society, there is a perceived immutable distinction between this man (whose traits were caused by transsexuality) and a cis man who has all the same traits (caused by other, independently-occurring disorders). The transsex man will be misidentified at birth as a woman, due to his disordered sexual development, but unlike another man (who happens to be cis) he won't be re-identified once he realizes the issue. And of course society does everything it can to prevent him from ever realizing at all.
The only distinction between cis and post-op trans is societal. The only reason to keep those words around at all, after the disorder is treated, is to describe the bigotry placed by society disproportionately on the "trans" man as opposed to the "cis" one. Ex: The only reason David Reimer is cis is because that's what society considers him to be.
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A man who doesn't have his dick goes to the doctor for a referral to a reconstructive surgeon. He either had a birth defect, or lost it in a car accident or when serving in the army. He can still piss and isn't bleeding out, but he's missing his entire genitalia. He doesn't have any medical records proving that whatever caused it really did happen.
The doctor takes a look at him, sees that he is in fact missing his dick and balls, and refers him to a phallo surgeon for reconstruction. Or—they do this if he's cis. If he's trans, they send him to a therapist to prove he really needs his dick. Then they send him to another, to get a second opinion—in case he regrets it. Really: they send him to a therapist because he isn't a man. A man missing his dick constitutes immediate surgical care, because that is a serious medical condition. A woman who wants to be a man needs three psychiatric approval letters to prove she won't sue them later when she chooses to change her gender again.
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A (cis) man who was infertile, had a mostly-nonfunctional micropenis and nondescended balls, and extreme hypogonadism and hormonal imbalance which (if not caught early) caused short height, wide hips, gyno, and other permanent skeletal effects, would immediately be classed as a serious medical condition.
He would need a prosthetic dick in the meantime before he could get reconstructive surgery, and would be distraught everytime he used the bathroom and over the fact he couldn't really have sex like he should be able to. He would have to take TRT for the rest of his life in order to treat his hormonal condition. He would need more reconstructive surgery if he wasn't able to catch the condition early enough to treat the damage it does during puberty.
Even after fully treating his condition, he wouldn't be able to get hard on his own with our current medical technology. He would never be able to father his own biological children. He'd have to worry about dealing with medication access and schedule for the rest of his life, and would have to keep track of his hormones in a way he'd never have to think about if he just had a normal daily hormone cycle.
Without treatment, his condition is usually fatal due to the amount of chronic pain it causes. He isn't able to have sex, to piss normally without (and sometimes even with) having to use a prosthetic. His body doesn't function properly because none of the hormone signaling is correct. I don't know what this would be if not a disability, and a fairly severe one at that. It's certainly more disabling than autism (at least, at the level I have it), which is acknowledged as a disability.
But the second that we call that same man "trans," it becomes a mental disorder.
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The same obviously goes for women but I know way more about the male medical stuff so thats what all my examples were