I never really liked that quote because of course you can define pornography. Just because it exists on a spectrum and there are grey areas doesn't mean you can't define it. That's the case for just about everything.

It's a lot like Loki's wager. We can't decide where the head ends and the neck begins, so these things must be outside the realm of science.

[1]: https://en.wikipedia.org/wiki/Loki%27s_Wager

Pornography isn't just a spectrum though it is? It requires a multi variable analysis and even then different people disagree a out what is or isn't pornographic.

Or is there some algorithm that will take a jpeg and 100% correctly tell us if it's porn or not?

The same is true of intelligence or free will.