If you want to wind people up, you could suggest Tim Williamson's epistemic solution to the problem of vagueness, which holds there are in fact hard, sharp boundaries, it's just we won't know quite where they are. It might appear sex is on a spectrum, but actually it isn't. 
It's basically invoking the Sorites paradox, & thinking what it shows is there are no people with full heads of hair and no bald people.
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Interesting: "a paradox that arises from vague predicates" (Wiki). Seems of some relevance, tho the "predicate" of sex only seems "vague" to those engaged in motivated reasoning due 2 vanity or envy. Which seems to include many feminist "philosophers":https://medium.com/@steersmann/reality-and-illusion-being-vs-identifying-as-77f9618b17c7 …
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