Yes, I am familiar with the Hijra, Berdache, Kathoey, etc. You forgot one: The Fa'afafine!
I said "taboo in most cultures". Not all. If you'd like to believe that the major cultures of the world in terms of population have and continue to embrace transgenders and homosexuals, be my guest!
The reptilian brain is responsible for our basic instincts (but since I 'm not a an expert in neuroscience, I may have used the wrong term :p) and yes, as stated, I submit a possible explanation as to why people in general do not embrace and champion transgenders ... and homosexuals for that matter, despite the huge progress made by the gay rights movement in recent decades.
You can say the explanation I brought forth is silly ... but it's not me that needs convincing, it's the non-supportive world.
Edit -
Many people have brought up the idea that bias exists in our society because the male privilege is compromised. Without getting into anthropological accuracy, surely the notion of male privilege is a construct of modern (ish) society? In primitive societies, wouldn't both men and women have combined their abilities in order to survive, there would have been no thought of a superior vs. an inferior gender, but rather just different bodies to fulfill different functions? And wouldn't a tribal member who could not contribute to everyone's welfare or survival be cast aside?
I'm making a mental note to read about primitive gender roles and survival practices. It's a fascinating topic.