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- Chromosomes
- Proteins
- Genitalia
- Gametes
- Hormones
- etc.
However, it's physically possible to end up with all sorts of non-binary combinations in this vector space. E.g.,
- Possession of a sex chromosome without expression of proteins
- Possession of a sex chromosome without corresponding sex hormones
- Possession of an irregular number of sex chromosomes
- Chimerism, where one fraction of your cells are XX and another fraction are XY
- Possession of neither or multiple genitalia
- etc.
Obviously it's possible to go too far in denying the clustering of two common sexes in this vector space, but you are making a basic factual error if you assert that sex is purely binary. There are all sorts of people who are intersex in various ways, and they are entirely real, not theoretical. And it gets far crazier if you look further afield in the animal kingdom, with species that can change sex and do all sorts of wild things.