Eeksteek: "Some people feel very masculine despite having a feminine body. Or vice versa. No biggie. (Which is how I interpret ‘being born in the wrong body’ @najene. You can’t really define someone who is the other sex in their own mind if you can’t define sex as a category anyway. I think I may have been responding to someone who quoted your post. Apologies for any confusion)"
Apology accepted. But this is not really any help. What is it to "feel very masculine despite having a feminine body," I wonder? Is it something like "to wrongly think one is a man when one is actually a woman"? Or perhaps, "to enjoy dressing and behaving in a way stereotypically associated with a man in a certain culture when one is a woman"? Or what?
The thing is, which I think you miss despite your claims to be focussed on scientific aspects, that the "born in the wrong body" notion is usually intended to play a causal role: this (senseless, I have argued in an earlier post) idea is offered in (causal) explanation of the supposed (and again senseless, I have suggested) state of being transgender (sc. being the opposite sex to that which one actually is). That your (Humpty-Dumptyesque) interpretation of the notion makes it unlikely to be able to fill such a role keeps it as empty of sense as it ever was.
Do you see?
As for "someone who is the other sex in their own mind". Do you really think this makes sense? Seriously? To be a certain sex in my own mind could at best be metaphorical for thinking I am a certain sex. But, well, everyone older than (roughly) two or three surely knows what sex they are? ... Oh, and so on: I leave this as an exercise for you to unpick for yourself.
... Do notice, though, how much definition is a red herring here. It's clear "someone who is the other sex in their own mind" has no (literal) sense. We can see this without calling up any scientific endeavour, or by looking in any dictionary ... all we need to be able to do is understand English at a fairly elementary level. (Notice that knowledge of definitions of words comes (a long way) after such understanding, both temporally/deveopmentally and (logico-)semantically.)
Of course you can define things however you like. But recall, perhaps, as well as Humpty Dumpty, Bertrand Russell's apophthegm about "the advantages of theft over honest toil." (To save you googling: “Introduction to Mathematical Philosophy, P. 71”.)
You'd do well to read a little around your own sparse knowledge of biological science, Eeksteek, if you really wish to understand where you are going so (badly) wrong on all this. I used to recommend Hull & Ruse's collection The Philosophy of Biology, but also I suspect you need something a tad simpler ... oh, go do some research for yourself.
(A little modesty might not go amiss, also, when posting here on MN: your de haut en bas schtick doesn't sit too well with what your posts tell us of your level of understanding.)