It's taken a lot of hard work to get to the point where homosexuality and transexuality (transexualism?) have been accepted at all, and transexuals especially are still at risk for being murdered.

I can think of two reasons why the political work has been done on sexual identity but not on species identity. One might be that people with marginalized sexual identities have more in common with each other than people with marginalized species identity have with each other, so organizing is easier for the former. The other is that various sexual identities aren't as distant from mainstream experience as various species identities, so what's being asked for is more comprehensible.

The work hasn't yet been done to convince any significant fraction of the general public that identity as a human is a thing which can be loosened up without disaster resulting, though I think the increase in respectability of science fiction and fantasy could be establishing a basis for the acceptance for otherkin.

I haven't heard of any sf set in a future where otherkin are accepted as people with normal variations of mind-- in other words, the tech for becoming an animal is no better than what we've got now, though plastic surgery is feasible and there might be some close-to-current tech sensory augmentation. Becoming a fantasy creature is visually easier, but other traits are mostly impossible. [1] I think that's a pretty cool premise.

This seems to be an argument that issues of publicly permissible identity are as much a political process as they are a rational look at a pre-existing universe.

[1] I talking with someone who found that the wolf ears on their Elfquest costume actually amplified sounds. It makes perfect sense-- all that matters is the shape-- but I was still delighted with the accidental intersection between fantasy and physics.