To save myself a lot of typing, mr-x, welcome to the internet, where there is information galore, just read up -
https://en.wikipedia.org/wiki/Gender
The better question, the one you keep skipping, is why are you so concerned with other people's lives? When I speak of Christian in this thread, I'm usually talking about the fundamentalist type. There are plenty of Christians that are decent and live their own lives. Then you have the other type, fudamentalist types. And there are different levels. Ones that don't believe in dinosaurs, ones that believe it's a choice to be gay, ones that believe in "pray the gay" away.
You are against gays being able to marry, but have yet to give me a real answer how it affects your life. It doesn't.
How does someone being born female, but doesn't feel that way and proceeds with that. This affects your life how? It doesn't.
So tell me what is wrong with the live and let live approach? Tell me why you're so concerned with other people, to the point you're ok with government intrusion into their lives?
How does other people and the way they choose to live their life, mr-x's business?