@EndoplasmicReticulum for you. Isn't it lucky that we aren't clownfish, after all. Poor old Nemo.
As to the rest - gender identity clinics in the early years did test patients for very rare DSDs, but found there was no overlap between trans and DSD individuals. Patients were male or female. (And actually the vast, vast majority of people with DSDs are very clearly and identifiably male or female, too - it's really ableist to say that eg a man with Klinefelter Syndrome isn't a man, for example.)
You can't change sex. Even if you had a DSD, you still couldn't change. You'd be what you were born as. And that's okay.
The real issue is not whether or not we have a biological sex, but if there is a second sort of innate gender identity as well, which may be different to that sex. There's no evidence at all to support that (even if you do think brains are sexed by biology and not socialisation - which is a very big if - it's rather odd to assert that the biochemical processes that perfectly created a sexed body would somehow leave the brain unaffected by those processes) other than the very strongly held beliefs of some people. And while those beliefs must absolutely be respected, it doesn't seem fair for the faith belief of one group to override the needs of another, who don't share the faith belief.
I don't want male bodies in single sex spaces. That's bad enough in communal changing rooms, but rape crisis centres, homelessness shelters, and domestic violence shelters are now allowing self identified males in. And then there's prisons. And yes, there have been rapes and sexual assaults. Plural. Why are biologically male people allowed to say, "I am a woman" and then be given all areas access, when we know 98% of sex offending is by men, and we also know that transwomen pose the same statistical risk as any other male? And why wouldn't they, really? Those words aren't magical. They don't mean Tinkerbell exists, and can transform male into female. Sex doesn't care what we say. It's immutable. It stays stubbornly constant.
Of course most transfolk are harmless. Most males are harmless. Not All Men Are Like That isn't the point. The issue is that enough are like that, and so few women are, that we have single sex provision to ensure our safety - and women in the developing world are still fighting for that, supported by all major charities. Single sex provision is safeguarding 101. That's the issue, not how some people, perfectly reasonably, want to live their lives. Go in peace and be happy, do whatever makes your life worthwhile. But don't eviscerate women's protections and provision while you're at it. That's all.