(In this post I try to stick to the sex-gender distinction, without which it is difficult to discuss sex ontology; if I trip up, I’d appreciate a correction)
Though they are often treated as the same question, trans rights do not depend on a particular sex ontology, and distinguishing between the claims makes supporting trans rights easier and helps to avoid demonizing people for legitimate ontological disagreements. No matter what one believes about the realities of human sexual categories, one can (and should) support trans rights based on general and broadly accepted individuals rights.
By “trans rights”, I am referring to the right of trans individuals to be recognized as their chosen gender, and treated by others as that gender for all social purposes. Defining it this way, and calling it “trans rights”, suggests that it is a right belonging only to trans people. It is more accurate to phrase it more generally, as the right of all individuals to assert, explicitly or implicitly, their gender, and have that assertion be respected by the people around them. Indeed, cis individuals exercise this right daily – many see it as quite offensive to imply that a cis man is a woman, or a cis woman is a man, and both implications are commonly used as insults.
But the right is made salient when someone asserts an identity that their community doesn’t expect, whether because of the individual’s historical assertions (i.e. they used to assert a different gender) or because their assertion is seen as in conflict with what others (i.e. they assert a gender different from what others believe about their sex). In those cases, the call to respect trans rights is the classic call to respect individual rights over the preferences of the community. As with any individual rights, trans rights are a limit on the power of the community to enforce conformity.
This right is well justified, and related to many other similar rights. The rights of privacy and bodily autonomy, for example: if the community refuses to accept that individual’s gender assertion because of a mistaken belief about their sex, the only recourse the individual would have is to submit to an invasive inspection of their anatomy, surrendering their body to the community for the community’s purposes.
Similar individual rights are at play in rights of conscience and the free exercise of religion: individual are permitted to refuse pork because their god proscribes it, even if the community does not believe in the same god. Beliefs about ones own identify are often as strong or stronger than religious beliefs, and demanding that a person adopt an identity at odds with those beliefs is at least as offensive to individual rights as demanding adherence to a specific religion.
And as with religion, showing evidence against a person’s beliefs doesn’t override individual rights. We shouldn’t force someone to acknowledge the age of the earth despite what we know from paleontology, and we shouldn’t force someone to identify as a specific gender, despite what we learn of their biology.
Finally, individuals’ rights to bodily autonomy extend to a right to do what they like with their own bodies, to modify them as they see fit. They can get tattoos and piercings, lengthen their legs, remove fat or insert padding, implant or remove hair, remodel their eyes or nose or chin as they like – the list is long and varied, and all are protected by an individual rights over one’s own body. So too should an individual be able to take whatever steps they wish to make their body conform to their beliefs about their gender, to shape it to assert the gender with which they identify.
These are not ontological claims about sex, they are claims of individual rights that do not depend on any particular sex ontology.
As with all individual rights, there are hard questions at the limit of where individual rights must cede to community needs. But as with all individual rights, we should have a strong presumption that the rights of the individual supersede the desires of the community. We should be reluctant to cede anything, and we should find ways to uphold rights to extent possible. These rights belong to us all, and recognizing and respecting them, especially when it’s hard, affirms the very notion of individual rights.