My post below elicited this response:
Here are a couple of cases which seem to defy easy classification. A “chimera”. This is a person who has cells derived from two zygotes. It can happen if two fertilized eggs merge very early in development. The individual may appear entirely normal (there may be chimeras reading this who are unaware of their condition); but the cells in their body will come from two quite distinct origins. If the original zygotes were male and female, then the adult individual will have some cells in their body with the XY (male) chromosomes, and others with the XX (female) chromosomes. There may be no external sexual ambiguity as long as the sex organs all come from the one lineage; in general all kinds of sexual ambiguity might arise. Second case; more common (though still unusual) is where an individual is genetically of one gender, and phenotypically of the other. This can be either an XX individual who develops with external male genitalia; or XY who develops with female genitalia. This is usually caused (I think) by excess or deficit of the appropriate hormones during fetal development. For most people, gender is unambiguous.
But there is no sharp dividing line or easy way of classifying that covers all individuals.
The examples I’m considering are entirely independent of psychology or choice. They are real physical conditions in which the conventional physical basis for determining gender becomes ambiguous.
First, I didn't really need that lecture. My post actually linked to androgen insensitivity syndrome. And I'm aware of other forms of inter/ambiguous sex, as noted above. And of course there are species where sex, as opposed to gender, is more fluid and facultative. I'm aware of all that. Rather, the issue hinges around the assertion that we must have a category system which covers all individuals. If that's your criterion than the vast majority of scientific concepts are social constructions with imperfect mapping. So, for example, people should legitimately attack the Endangered Species Act as being grounded in fuzzy science (the species concept). We shouldn't talk of planets and asteroids, where do you draw the line? And so forth. If you assume that sex determination occurs through inheritance of sex chromosomes, the distribution above for sex diagnostic characteristics should be expected. You can generate a synthetic metric, and males and females should cluster together. But you're going to have developmental and chromosomal abnormalities, with the latter leading to a third mode in between the male and female. I wouldn't make much of this argument, but I've encountered a rejection of the sex concept from biologists. Which begs the question: what the hell are evolutionary biologists studying then when it comes to sex? Obviously the last is a silly question, everyone knows what biologists are studying. It doesn't matter that sex is not a concept which exhibits the perfect clarity and precision of a mathematical proof. It's about as good as it gets in biology for an abstract category. We aren't sure how to define life itself! That means biology in its own foundation doesn't exhibit the stringency some require from sex.