‘Transgender’ Could Be Defined Out of Existence* I doubt there are that many transgender people under the authority of the US that this is a big practical issue for the American State or People. The meanness and pettiness of the Trump administration here — marketed as conservative toughness — is probably the point.
* Usage has usefully evolved to where "sex" can refer to biological sex, starting with XX people and XY people. That can get complicated on just the level of chromosomes and intrauterine environment, but, roughly, H. sap. is a "sexually dimorphic" species with "the modal phenotype" bimodal: most of us are born and live sexually male or female. "Gender," in the useful evolving usage, has been sex as socially and culturally manifested — and that can get very complicated and go way beyond XX people and XY people and "innies" and "outies."
So far as they're thinking at all (as opposed to scheming and calculating), the Trump administration is making the same sort of mistake as the Roman Catholic. Church on contraception. They're not thinking "nature" and "natural law" in a modern scientific sense buy "Natural Law" in the sense of Aristotle and a simplistic idea of a purpose for "sex" in at least two senses: sex in the sense of the sex of an individual, sex in the sense of the activity.
Very generally, this is the same sort of problem ethologists — behavioral biologists — could see with Trump's loose use of "instinct." There's no "hunting instinct" in, say, cats, and the purpose of feline hunting isn't eating. There's a hierarchy of "species-specific, genetically-encoded behaviors" — spotting, tracking, stalking, pouncing, biting, shaking, eating — that in nature have cats hunting, and usually eating, very well, thank you. Similarly with most human sex: Coupling for the purpose of reproduction is relatively rare. Many people copulate and there are enough births that we have (re)filled the Earth and kind of subdued it and can slow down already with the reproduction and "dominion" stuff. For political arguments for such subjects START HERE, and then sex and gender issues become intellectually wonderfully complicated — and the political issues get down to courtesy and accommodation to reasonable requests/demands and just telling small sexual/gender minorities making unreasonable demands — as with any interest group — to f*ck-off. The Trumpian attempt to define away the gender issues and the transgendered and avoid useful political conflict, is not only mean and petty, but also cowardly.