There is however a concurrence of trans IDing in twins that suggests it is more than just coincidental.
Okay. So if it was an innate essence (or internal sense of identity), if one twin had it, the other one would have to have it. They have the same genetic make up. The fact that there is a coincidence amongst twins, only shores up the nurture argument, not the nature.
They would have been subjected to the same environmental circumstances. Outside influences would have played a part, and their individual experience may have been just different enough to make one of them trans and one of them not.
There may be be a neurological blip that provides a propensity for things like trans - e.g., general body dysmorphia, transablism, anorexia, otherkin, etc. So a possible proclivity for some kind of disassociative disorder, which requires circumstance to fulfil it.
You get plenty of effeminate men, and plenty of masculine women.
There have been brain scans, MRIs, all sorts of studies, which are still inconclusive.
So even if a man was found to have had something that made him more effeminate (whatever that is, I can't even think), he'd still be a man.
This idea that it is your brain that makes you one sex or the other and not your body doesn't hold water. Because it is your body, that gives you the lived experience of being that sex. Both in actual biological terms and the way the world treats you.
In terms of being non-binary, it's interesting, but I definitely think it's a social preference. You may not feel completely masculine or completely feminine but, hello, welcome to the human race.
Getting people to accommodate you in this, is something else.
People don't care. And the more you try to make it significant, the more they think you are driven by ego and self interest. And they go from not caring, to being annoyed.
It can only possibly be motivated by how you want other people to treat you. Otherwise you wouldn't need a label and a fanfare. You would just live it, irrespective.
They're this close to the idea of abolishing gender, but mess it all up by making it all about how they are treated, and not how they treat others. If they started a movement with they treated everyone else as genderless, then we'd be getting somewhere.