There exists a reasonably-well-understood concept of the fallacy of the excluded middle, or false dilemma. But what do you call it when it's not the middle which is excluded but the two ends? Is there a word for the logical fallacy whereby one argues from "There are cases where it is difficult/impossible to decide whether something falls into category A or category B" to "Nothing can be confidently stated to be either A or B" and/or "A and B are useless/meaningless categories"?
One of the many irritating manifestations of this is a kind of childish what-iffery. "But WHAT IF there was a case where there were three identical twin sisters who were respectively married to three non-identical twin brothers and they were all gay and all had different fatal diseases but only two of the brothers and a non-corresponding two of the sisters were in a higher tax-band and the plane all six of them were in crashed ON THE INTERNATIONAL DATELINE? Would that count?"
Is sorrel goth? IAMFI.
On re-reading I'm not sure I agree any more that the what-iffery is quite the same thing, although I can see how I got there (both are a way of trying to insist that because there are edge cases there can't be any useful generalisations). I'm struggling to think of a better example that doesn't involve gender/sex/sexuality (because talking about any of these things on the internet just makes people angry).
Relatedly, I had another fragment somewhere about 'conversations which I really hate getting into' but I think I deleted it already. It wasn't even about the dangerous conversation topics, just the tedious ones. The topic of forbidden topics is something I keep circling around and not wanting to address; in some ways I think the meta-conversation is even more risky than the conversations themselves. I have a different version of that argument in a notebook somewhere -- when I get round to digging out the 'fragments' on paper perhaps I won't feel the need to avoid it again. When I grow up. Maybe.