But that's not what you said, you said "Because skin colour is purely a matter of melanin. Gender is much more complex than that and involves hormones, brain chemistry, brain structure, and the intersection of all those things with psychology".
I'm very keen to read the science behind skin colour just being purely about melanin (seems a bit dismissive of entire cultures and ancestry just quietly), and how this inner gender interacts with "brain chemistry" and structure and stuff.
On the topic of biology, I'm also quite keen to hear what transactivists make of recent research that shows the cellular history that is stored throughout the matralinial line, i.e. where mothers retain some of the cells of their offspring for their entire lives, and also the profound yet barely understood interaction between a baby's growth and the quality and nutrient components of mothers breastmilk over time. And, I can't remember the exact details (not a biologist, just friends with one) but isn't there some circumstance where a woman, maybe following pregnancy, can't donate blood to men because it causes some kind of potentially fatal reaction?
But yeah, he is a girl 'cause of "brain chemistry" and "psychology", sure.