It's biology. Evolutionary biology to be exact.
'Trans' appears to me to be a purely sociological and psychological phenomenon. Not biological, and certainly not supported by evolution.
Males and Females are a biological phenomenon. They exist because they work. Males who think they are females (or vice-versa) is either psychological or sociological in origin. Both things requiring an animal with a higher functioning brain (aka humans).
'Trans' as a thing does not work in evolution. If there were Trans birds, birds would no longer exist.
There are no Trans fish. But there are Male fish and Female fish.
I could 'wall-of-text' this subject but I think I've made my point. Remember, I asked Vanessa to read Origin of Species for a reason. Hodge thought it was some sort of Atheist argument but it was deeper than that.
TL;DR: Biology should be a pre-requisite of Sociology and Psychology.
If you go down the rabbit hole of Sociology and Psychology without having fist done at least Biology and Chemistry, then you are completely and absolutely lost.