What is it about gender identity that separates it from fringe subcultures like otherkin, soulbonders, and whatever else?

Transsexuals have found a way to integrate themselves to the point that they can "pass" as their chosen identity, and that pretty much forced society to deal with it, either by declaring them "fakes" or by accepting them. Otherkin don't seem to generally suffer nearly the same degree of dysphoria, and can't presently manage to "pass" as a dragon.

The idea that gender is intrinsically derived from sex is not culturally universal, either. Quite a few cultures have male-bodied females, or female-bodied men. In most major English-speaking countries, you have to undergo transition if you're a male-bodied female and want to be respected as such.