I agree it is a pretty sharp boundary, for all the obvious evolutionary reasons - nevertheless, there are a significant number of actual really-existing humans who are intersex/transgender. This is also not too surprising, given that evolution is a messy process. In addition to the causal structure of sexual selection and the evolution of humans, there are also causal structures in how sex is implemented, and in some cases, it can be useful to distinguish based on these instead.

For example, you could distinguish between karyotype (XX, XY but also XYY, XXY, XXX, X0 and several others), genotype (e.g. mutations on SRY or AR genes), and phenotypes, like reproductive organs, hormonal levels, various secondary sexual characteristics (e.g. breasts, skin texture, bone density, facial structure, fat distribution, digit ratio) , mental/personality differences (like sexuality, dominance, spatial orientation reasoning, nurturing personality, grey/white matter ratio, risk aversion), etc...