Biological sex is determined by the sex cells produced. There is no non-binary sex cell.
So non-binary sex cells determine non-binary biological sexed qualities. You still can't say all cismen have entirely male bodies. All biological types are varieties, they involve members that differ from individual to individual. So sex is not an essence, but rather a plurality of different qualities that vary from one individual to the next, and an individual can have sexed qualities from a sex different from their own, e.g. a cisman can have female sexed qualities.
But I do consider this an advance because you're proposing an objective criterion: if a man came to malelesbian MD saying he was a 'man with gynecomastia' you would say, 'No, gynecomastia means you're non-binary and you've no say in my diagnosis of your sex/gender because science'
False. I would still say the gynecomastic patient was a man, because he identifies as a man. The only objective criterion I propose is that if a person identifies as a member of a particular gender, I will accept them as a member of this gender. As far as I can tell, only transphobes disagree with my criterion. In my pro-queer view, only the individual person identifying as, in this case, a man, has any say in my identifying him as a man. Science lacks any authority in a person's gender identifications. Science does have the authority to assign a sex to a person, but as we've seen, a person's sexed qualities can differ from their assigned sex.
Furthermore, science invented gender reassignment surgery, which shows that even the sex assigned to a person at birth can change, and is far from immutable, as a certain rageaholic claimed earlier in a manner quite contrary to his avowed anti-essentialism.
Meanwhile, I'll continue affirming anti-essentialism about biological sex.