Do you have a particular edge case in mind? One that’s commonly brought up is en.wikipedia.org/wiki/Ovotesticular_syndrome, but that doesn’t fall outside the sex binary. Having a bit of nonfunctional tissue doesn’t affect one’s sex.
Colors aren’t a great analogy either, because in anisogamous species, gametes are strictly binary. There’s sperm and ova, with 0 overlap and 0 other options. “Purple gametes” just don’t exist.
This also isn’t my opinion, this is the accepted definition in the field of biology.
ewigkaiwelo@lemmy.world 6 hours ago
It is binary if you define it as a concept of reproduction, which it is. Every human that has ever been born had two parents. Looks pretty binary. Intersex people and edge cases cannot reproduce naturally, to my knowledge. So only those belonging to those two specific biological categories of sexual reproduction (males & females) can reproduce.
ewigkaiwelo@lemmy.world 6 hours ago
Also sex is defined by a pair of chromosomes, at conseption sex cells merge, each of them has only 23 chromosomes, the resulting cell has 46, which is why they say that sex is defined at conception, you can’t determine the sex at this stage without destroying the single cell (zygote), but the chromosomes will not change nevertheless