Thanks! I hope you pursue it further. It's a lot easier now than 5, 15 or 50 years ago.
Hirschfeld's activism was set in motion when he realized the gay bars of Chicago were very similar to the gay bars he knew back home in Germany. Which yes, does suggest way more questions than answers are available. From this, combined with his training in science, he derived a hypothesis of sameyness of humans when it came to sex. (There are gay people everywhere.) He soon extended this to trans people and to many others. After working for a while on reparative therapy, he gave it up as hopeless, falling back on the understanding of Darwinism as alluded to above. If beaks of Galapogos finches can be illustrated as this, it obviously follows sex/gender would be the same:

All this hateful, bigot shit, all of it is man-made. It's not natural to be ignorant of, or to hate, any kind of person, just as a bird holds no hostility to a beak shaped one way or another. All the shit we deal with is downstream of (admittedly successful) interventions by fascists. They make their hatreds appear natural which is a total lie. GSM have always existed wherever on earth in every population. While understanding that all modern constructs are products of modern material conditions, we can also see ourselves: all kinds, every where, every time.
Per Scientiam ad Justitiam = Through Science, To Justice