Those are anomalies and imperfections like extra toes and fingers. It does not mean that there's sex identifications for every difference. Sex biology is as basic as the birds and the bees and has existed since the dawn of humans.
Transvestite studies was researched in the 1800s, early 1900s. Transgender field is fairly new, created in the 1960s by social scientists, and the definitions keep changing.
Someone with male chromosomes and a ding dong who can reproduce.
And don't come up again with ''there are millions who are not XX or XY'' - they are the minority and not the average or ''norm'' if you wanna call it like that.
There are exceptions to everything.
There are alot of anomalies, but that doesn't mean every single difference changes sex or gender.
You can be a woman with high testosterone, that doesn't mean you are a man.
Look, as a scientist myself I have to stop you here.
You're vastly overstating the overall importance of the topic.
About 0,2% of the overall population don't fit into the very basic biological male-female classification (which we call intersexual people).
99,8% of the overall population can be biologically identified simply by looking under their pants (phrasing it drastically).
Even if you look at the percentage that defines itself as "man" or "woman", while being the opposite biologically (trans-people), we're still at a rate below 1% that doesn't fit the standard biological factors to their preferred gender.
So "basic biology", which you somehow seem to dislike, perfectly describes more than 99% of all cases, which is actually a sensational number for us scientists.
I'm addressing the three of you at the same time because you all went for a probability argument that I find to be pointless. It's irrelevant whether something is a minority and doesn't represent the norm —it still exists and therefore we must take it into account in order to paint a full picture of an issue. If we do the contrary and rule out the cases which won't fit the male-female dichotomy and/or show sex-gender incongruity, then we will be staying at a surface understanding of this issue, which is utterly antiscientific.
Trans population hardly reaches the 0,5%, if I recall correctly. It's minoritarian by definition, but it doesn't mean their condition is less legitimate, less real, less important; which is why it's still researched by a wide range of fields from neuroanatomy to endocrinology, not just social sciences as one of you insists on stating. Anything other than fully understanding why these people exist and what defines gender and sex beyond what's "under their pants" is indeed basic biology, which I repeat, is bullshit because no scientific field is basic but filled with nuances, exceptions, rarities... that aren't less meaningful than the norm just because they are less.
So, to put it simple, I'm not overstating the importance of anything. I'm stating the fact that people beyond XX and XY exist; the fact that other factors apart from what's under your pants must be and are in fact being studied in order to understand sex and gender; the fact that people exist who experience an incongruity between their sex and gender which is defining their lives in a very obstaculizing way yet can't simply choose not to be like that. And I couldn't care less whether it's 50% or 0,5% of the population because reality isn't about porcentages but phenomena.
And as long as one piece of phenomena contradicts what was previously established about a construct, then science must address it to redefine such construct. Just like we won't defend that swans are white birds just because black swans are a negligible oddity.