There are only two sexes because there are only two gametes
Helen Joyce said:
“There are only 2 sexes. And the reason there are only 2 sexes is because there are only 2 gametes, and the gametes are the thing that define the sexes. This is established in biology.”
Gametes are a real and well-understood biological category, and the claim that sex is defined by them sounds like a straightforward appeal to established science. Many people will have learned in school that reproduction involves eggs and sperm, so anchoring sex to gametes feels rigorous.
Gamete type is a real category in reproductive biology, and nobody disputes that eggs and sperm exist. But a definition that works at the level of a species' reproductive system is not the same as a property that maps cleanly onto every individual human body. Roughly one in fifty people are intersex in some way, many people are infertile, many produce neither gamete type in the course of their lives, and none of that makes their sex ambiguous in everyday terms: it just shows that the gamete definition was built to describe reproduction, not people. Biologists themselves actively debate how to define sex at the level of the individual organism, which is the opposite of a settled question, and presenting one framework from one subdiscipline as the whole of established biology is what does the political work here. The people who pay for that sleight of hand are the ones whose bodies or identities do not fit a definition retrofitted to exclude them.
What I said at the time:
“To say that the whole of humanity is to be dictated by whether your body produces eggs or sperm is so so mathematical, so binary, so basic.”
Intersex people and trans people whose existence is erased or pathologised when a single reductive definition is treated as the whole of biological science.