Short answer: Mu.
Longer answer: "Porn" is clearly underspecified, and to make matters worse there's no single person or interest group that we can try to please with our solution: many different groups (religious traditionalists, radical feminists, /r/nofap...) dislike it for different and often conflicting reasons. This wouldn't be such a problem -- it's probably possible to come up with a definition broad enough to satisfy all parties' appetites for social control, distasteful as such a thing is to me -- except that we're also trying to leave "eroticism" alone. Given that additional constraint, we can't possibly satisfy everyone; the conflicting parties' decision boundaries differ too much.
We could then come up with some kind of quantification scheme -- show questionable media to a sample of the various stakeholders, for example -- and try to satisfy as many people as possible. That's probably the least-bad way of solving the problem as stated, and we can make it as finely grained as we have money for. It's also one that's actually implemented in practice -- the MPAA ratings board works more or less like this. Note however that it still pisses a lot of people off.
I think a better approach, however, would be to abandon the question as stated and try to solve the problem behind it. None of the stakeholders actually care about banning media-labeled-porn (unless they're just trying to win points by playing on negative emotional valence, a phenomenon I'll contemptuously ignore); instead, they have different social agendas that they're trying to serve by banning some subset of media with that label. Social conservatives want to limit perceived erosion of traditional propriety mores and may see open sexuality as sinful; radical feminists want to reduce what they see as exploitative conditions in the industry and to eliminate media they perceive as objectifying women; /r/nofap wants what it says on the tin.
Depending on the specifics of these objections, we can make interventions a lot more effective and less expensive than varying the exact criteria of a ban: we might be able to satisfy /r/nofap and some conservatives, for example, by instituting an opt-out process by which individuals could voluntarily and verifiably bar themselves from purchasing prurient media (or accessing websites, with the help of a friendly ISP). If we have a little more latitude, we could even look at these agendas and the reasoning behind them, see if they're actually well-founded and well-targeted, and ignore them if not.
This all sounds reasonable to me. Now what happens when you apply the same reasoning to Friendly AI?
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?