I think there is no fundamental difference between porn and erotica, it's just that one is low status and the other is high status (and what's perceived as highs status depends greatly on the general social milieu so it's hard to give any kind of stand-alone definition to delineate the two). It only seems like there are two "clusters in thingspace" because people tend to optimize their erotic productions to either maximize arousal or maximize status, without much in between (unless there is censorship involved, in which case you might get shows that are optimized to just barely pass censorship). Unfortunately I don't think this answer helps much with building FAI.