You're right, it's not your genitals, you could lop mine off, and I would still relate to you as a man. It is something much deeper within us, something we don't yet know how to change. Being gay or trans doesn't change that fundamental thing, it's a different approach, but it's still not different. You can put a trans-woman in front of me, and I promise you, I would relate to her as a man, on that intrinsic level which is so hard to define. (This is kind of a dumb point in a text based conversation, as I really have no proof, but I do think you (as are others) are ignoring my point. How we think is very much related to how we are made).
I seriously doubt that's true. Chances are high that you've already interacted with a trans person and not even known about it. The point people are making is that that fundamental thing that you can't change is actually biologically based, but it's a brain thing, not dependent on genitalia or chromosomes.