[aaaja]

> exists on a spectrum, rather than being strictly binary

Not really though, at least not for biologists, who have an understanding of sex that encompasses all sexually reproducing species, based on there being two gamete types of different sizes.

This is why it's described as a binary, because of the two distinct types of reproductive cell. By convention the larger type is the female gamete and the smaller type is the male.

Having these consistent definitions is how biologists can discover new sexually reproducing species and understand the specifics, like are individuals hermaphroditic (each individual produces or can produce both female and male gametes) or gonochoric (individuals with distinct and unchanging sexes), and if the latter, which are the females and which are the males.