> but that doesn't mean I don't understand what porn is
You only understand what it is for you. The history of complete failures with regard to obscenity laws makes it quite clear though that this is not sufficiently common to produce satisfactory definitions at a societal level.
You are mistaking the overlap of opinion you share within your social cohort with some sort of objective, species wide truth.
As I said, we don't understand such things at a deep enough level to come up with formal models of them. But we do know what it is that we'd want to model. We do know for sure that humans and other mammals are sentient, while rocks are not.
I also can't generate a black box test that will predict whether something is porn, but that doesn't mean I don't understand what porn is, or that it is some deep mystery.