There is a qualitative difference between being XY and X0 (Turner’s syndrome). Being XY, a guy, is the physical embodiment of an evolutionary strategy: a certain genetic pattern that has a way of making more copies of itself. It’s a complex strategy, but that’s what it is. X0 people are sterile: they don’t generate more X0 individuals. Not every XY individual succeeds in reproducing, any more than every maple seed starts a new maple tree – but on average, it works. An X0 individual is the result of noise, errors in meiosis: Turner’s syndrome isn’t a strategy. In the same way, someone with Down’s syndrome isn’t Nature’s way of producing more people with Down’s syndrome.
Parenthetically, being a guy that tries to reproduce is a strategy. Actually reproducing is a successful outcome of that strategy. Similarly, being an alpha dude in a polygynous species like elephant seals is not a strategy: trying to be an alpha dude is the strategy. I see people confuse those two things all the time.
In eusocial species, like ants and bees and naked mole-rats, evolutionary strategies include individuals that seldom or never reproduce – worker and soldier castes. They still contribute to the reproduction of close relatives. Are humans eusocial? Obviously not.
A leper is an example of someone that’s been heavily influenced by the reproductive strategy of some other species.
Natural selection tends to make physical embodiments of a successful reproductive strategy common. So stuff like Down’s syndrome, Turner’s syndrome, androgen insensitivity, etc, are all rare. Successful evolutionary strategies usually involve actually getting things done: so there is a tendency for natural selection to develop and optimize various useful abilities, like walking and talking and thinking. All part of the strategy. Many non-strategies [like Downs or Fragile X] mess up those abilities
How many such strategies are there in humans? At least two, XX and XY. Guys and dolls. Are there are any others? Logically, there could be. For example, some species have two or more male morphs, embodying different strategies. Most male salmon go to sea, try to survive and get big, eventually coming back to freshwater to spawn. But some [jack salmon] become sexually mature when younger, after a shorter time at sea. They’re smaller, but less likely to get eaten by sharks.
Is there any evidence for alternate evolutionary strategies in humans, other than just male and female? Not really, so far. For example, schizophrenia looks more like noise, sand in the gears. Not much of the schiz genetic variance shows up in GWAS samples: it looks like it’s mostly driven by rare variants – genetic load. There may actually be some truth to the notion that happy families are all alike.
So, is sex a spectrum in humans? No: obviously not. Two basic strategies, plus errors.
Why would a geneticist be unable to make the distinction between an evolutionary strategy and an error of development (i.e. caused by replication errors or pathogens)? Well, the average geneticist doesn’t know much evolutionary biology. And being embedded in a university, the current replacement for old-fashioned booby hatches, he’s subject to pressures that reward him for saying stupid things. and of course some people are pre-adapted to saying stupid things.