I'm very liberal, too, but transgender is NOT a sexual orientation. It is a psychological disorder. The person does not like who they are and believe that being someone else will solve their problems. It won't. They'll just change, realize this and be more miserable than ever. I could hate myself as a human and believe that I'm actually a tiger. (No one would argue that that is a psychological disorder!) And yet painting stripes on my face and gluing whiskers to my cheeks won't make me a tiger. It would just make me a very odd looking man. And I would be crushed, even suicidal that the other tigers did not accept me for what I wanted to be.
Homosexuality is utterly and completely different. In fact, the famous trans, Bruce Jenner, likes women. He's not gay. He's a straight guy that dresses in women's clothes.