That’s not them defaulting to whatever you think their actual biology is. (Being trans may indeed be biologically based – just not in the same direction as the person’s genitals are.) That’s them being misgendered by other people. And in a particularly nasty fashion.
Nothing final about it, with modern medical technology.
And to the extent that trans people feel they can’t escape the portions of their bodies that don’t agree with their gender: isn’t that one of the causes of suicides? (Plus the attitude of society, of course; even when not expressed in the form of pointing out that one’s body may be more likely to be raped.)
I think, that it’s whatever it was that caused me to be entirely certain that I was a girl who wanted to do some of the boy things and not to do some of the girl things, including well before I had any idea that there was more than one type of genitals. (1950’s, no brothers, no male dogs. Neutered male cats don’t show anything obvious.) My reaction to being told that I was “walking like a boy” was “I’m a girl and I walk like this, so this must be a way that girls walk.”
Apparently not everybody has that sense of rock solid certainty of their gender. And a lot of people who do have it are cis. But I have no reason to doubt that some trans people, children included, have that same sense; and that this is why they’re insistent that their gender doesn’t match their plumbing.
What’s interesting is that that became stricter, not looser, as opportunities for women opened up.
I’ve got a 1950’s Sears catalog. The toy pages aren’t segregated. A number of the toys are clearly shown as for both girls and boys. The ones that aren’t relied, not on separate pages or color coding (there was very little color coding of anything), but on the assumption that of course everybody knew the nurse kit was for girls and the doctor kit was for boys – but the two were right next to each other on the page.