Transgendered people haven't been around for thousands. People that have defied the typical Western gender binary yeah, but people cutting off their penises or giving themselves penises, no. People have cross dressed or embraced androgyny forever, but give themselves a damn medical condition.
Transracialism or transethnic identities got more validity anyway. What do you call Hellenistic Egypt, when the Greeks came to Egypt and adopted their gods and culture and even became rulers of Egypt?
People have came across cultures and ethnic groups that they admire and adopted their culture as their own forever.
Not too many people have surgically altered their body and reinforce the gender binary.
Only thing transgenderism got behind it is pity and filmsy medical science because we feel bad for them. Not because it's a valid thing, it doesn't pass the crazy test.
Shit, humans been fucking animals for thousands of years yet we don't make it a valid option and there's science behind that too, we don't say, "Yeah, go ahead and fuck your dog."