Maybe another way to put this is that two white people can have a baby. That baby can be male/boy, female/girl, can be intersex, can be somewhere in between. And it's perhaps reasonable to believe that somewhere along the line, something didn't quite line up the way that person feels they should.

But

Two white people cannot make a black baby. They cannot because their parents were not black, who can't because their parents were not black. Etc.

Like I said, I'm still grappling with this and it's articulation. But I think what I'm getting at is perhaps that gender identity is intrinsic or that it comes solely from within a person. There is a piece of racial identity that is instrinsic and comes from within, but a whole lot more of it is historical "baggage" so to speak. Race is expression of those two things and therefore isn't as fluid.