Someone with XXY chromosomes, assuming a functional SRY gene and the genes downstream from that, will be male. Klinefelter's syndrome is a condition that only a

Someone with XXY chromosomes, assuming a functional SRY gene and the genes downstream from that, will be male. Klinefelter's syndrome is a condition that only applies to males. The biological indicator of sex isn't the chromosomal configuration - that's just a proxy. The indicator is which pathway of sexual differentiation was triggered during development. Sex is a binary because that's how anisogamic sexual reproduction works: females embodying the reproductive role organised around large gamete production, and for males, small gametes. This still accounts for disorders of sexual development too: a male with underdeveloped testes is still male, and a female with non-functional ovaries is still female. In some extremely rare occurrences of chimerism, a person could reasonably be said to embody both. But that's not what anyone means when they use the term "non-binary" as an identity - it's expressed by people who are unambiguously male or female, but who feel that they don't fit into the cultural gender stereotypes of masculinity or femininity.