Do you have a particular edge case in mind? One that’s commonly brought up is https://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.

This is often a point of confusion, but human sex is binary. There’s edge cases that require clarification as to how they fit into the binary, but don’t disprove it.

Human sexuality overall is complex and that’s why we differentiate gender from sex. The sex binary and gender spectrum complement each other though, and don’t clash.

If you’re interested in learning more, here’s some background reading:

https://en.wikipedia.org/wiki/Gonochorism

We fall into that category, where we have two body plans, each organized around producing either sperm or ova. Other species have more body plans, such as recognizably distinct males, females, and hermaphrodites:

https://en.wikipedia.org/wiki/Trioecy

Those species are a good contrast. Humans don’t have that variation, and so sex is binary in humans.

There’s literature that explains this specifically in detail, though most of it doesn’t really explicitly talk about it, much like math papers don’t generally explain that integers can be added together.

I think we agree. Maybe I was misreading the above comment, but I was just clarifying that “transform” in that sense for chickens is not actually changing sex, and so is a different situation than clownfish.

I’m not aware of that being an actual change in sex. The hen can develop male characteristics, but won’t produce functional sperm.

Male seahorse get pregnant, but that doesn’t make them trans, they’re unambiguously male. This is a great example of why sex is defined by gamete size. If it weren’t, we couldn’t talk about males and females in any useful way across the animal kingdom.

Clownfish would be a better example as they’re sequential hermaphrodites, but that doesn’t have any bearing on the human sex binary.