bilby
Fair dinkum thinkum
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All of which is true in general, but not in specific cases.Sex is binary in all anisogamous species. That includes all mammals, all birds, and the overwhelming majority of vertebrates.I guess the bit about being binary is incorrect
Sex is defined based on the type or reproductive system that an individual develops. This stems from the fact that within species that reproduce sexually, there are only two types of gametes. Large sessile gametes are eggs, small motile gametes are sperm. Sexual reproduction occurs when two differently sized gametes merge and exchange DNA to create offspring.
Anisogamy developed a few hundred million years ago in our evolutionary lineage. Part of that evolutionary process also led to individuals within each species evolving reproductive systems that support the production of those two types of gametes. Across all species - including a lot of plants, we can observe two types of reproductive systems, although the formation of those systems differs from one species to another. The commonality is in the function of those systems. Within any given species, the set of individuals who have the system that has evolved to support the production and delivery of small motile gametes are called males. Those who have the system that evolved to support the production of large sessile gametes are called females. Note that the definition is dependent on the type of system that the individual develops, and which function that system evolved in concert with. It does not require that the individuals actually produce any gametes at all, nor does it require that the entire system is present and functional. Thus a prepubertal female is still female, even though she doesn't yet release mature eggs. And a male who has lost his testes through accident, illness, or intent is still a male even though he can no longer produce sperm.
For sex to be something other than binary, there would need to be an evolved reproductive system that supports the production of a different type of gamete. To date, there is no other type of gamete among humans - only sperm and eggs. Thus, sex in humans is strictly binary.
If you say that humans are bipeds, and so have two legs, that is true in general. But it does not imply that someone who claims to have only one leg is lying, mistaken, or wrong.
Sex in humans is binary. Humans have two legs. Humans have five digits on each hand. All facts. All indisputable and backed by both observation and our best current theories of human development. And all untrue for a small but non-zero number of individuals.
Exceptions exist. If there are two developmental paths, then that immediately implies at least four outcomes: A, B, AB, and 0. Nobody is daft enough to declare that people of blood type AB don't exist - even though they are very rarely encountered (and mostly you wouldn't know you had encountered one unless they decided to tell you). Yet you are here insisting that people who are both male and female, or who are neither male nor female, do not and cannot possibly exist. Which is absurd. And is based on the misapplication of a general rule, assuming (incorrectly) that because that rule applies in the majority of cases, it must apply to every single specific case.
Six fingered men exist, despite the strong evolutionary pressure against their existence due to their being hunted down and killed by Inigo Montoya.