> Do we actually know that yet though? Handedness seems to be genetic, but for the rest we really don't know the causes.

Yeah we know that (for various definitions of "know", but the evidence suggests biological factors have the most impact). There could be more research on that topic (and there would have been if the Nazis didn't burn down the first research institute that researched that field). This is a good starting point: https://en.m.wikipedia.org/wiki/Causes_of_gender_incongruenc...

> Why? It shouldn't matter if it's innate, a development 'disorder', a choice, or a result of trauma or social contagion. We can just accept people anyway.

Then we agree. And I am not sure "The" left wouldn't agree with the both of us as well. Most leftists I know are also pretty accepting of similar conditions (e.g. mental health acceptance, etc).