A couple of thoughts here:
Set a high minimum price for anything arousing (say $1000 a ticket). If it survives in the market at that price, it is erotica; if it doesn't, it was porn. This also works for $1000 paintings and sculptures (erotica) compared to $1 magazines (porn).
Ban anything that is highly arousing for males but not generally liked by females. Variants on this: require an all-female board of censors; or invite established couples to view items together, and then question them separately (if they both liked it, it's erotica). Train the AI on examples until it can classify independently of the board or couples.
Set a high minimum price for anything arousing (say $1000 a ticket). If it survives in the market at that price, it is erotica; if it doesn't, it was porn. This also works for $1000 paintings and sculptures (erotica) compared to $1 magazines (porn).
I doubt that that works. What makes you think there are no rich guys who want to see pornography? They will simply buy it at the $1000 price.
I can think of no reason why price discrimination would favor "art" over porn.
To construct a friendly AI, you need to be able to make vague concepts crystal clear, cutting reality at the joints when those joints are obscure and fractal - and them implement a system that implements that cut.
There are lots of suggestions on how to do this, and a lot of work in the area. But having been over the same turf again and again, it's possible we've got a bit stuck in a rut. So to generate new suggestions, I'm proposing that we look at a vaguely analogous but distinctly different question: how would you ban porn?
Suppose you're put in change of some government and/or legal system, and you need to ban pornography, and see that the ban is implemented. Pornography is the problem, not eroticism. So a lonely lower-class guy wanking off to "Fuck Slaves of the Caribbean XIV" in a Pussycat Theatre is completely off. But a middle-class couple experiencing a delicious frisson when they see a nude version of "Pirates of Penzance" at the Met is perfectly fine - commendable, even.
The distinction between the two case is certainly not easy to spell out, and many are reduced to saying the equivalent of "I know it when I see it" when defining pornography. In terms of AI, this is equivalent with "value loading": refining the AI's values through interactions with human decision makers, who answer questions about edge cases and examples and serve as "learned judges" for the AI's concepts. But suppose that approach was not available to you - what methods would you implement to distinguish between pornography and eroticism, and ban one but not the other? Sufficiently clear that a scriptwriter would know exactly what they need to cut or add to a movie in order to move it from one category to the other? What if the nude "Pirates of of Penzance" was at a Pussycat Theatre and "Fuck Slaves of the Caribbean XIV" was at the Met?
To get maximal creativity, it's best to ignore the ultimate aim of the exercise (to find inspirations for methods that could be adapted to AI) and just focus on the problem itself. Is it even possible to get a reasonable solution to this question - a question much simpler than designing a FAI?