The difference between art and porn is *most* people respect art, and *most* don't respect porn. There is no objective test for art, porn, pretty, ugly, useful, obsolete, worth, or even good and evil (e.g., is a prey animal gulping up a defenseless hatchling evil?).
The term art and porn desribe people's reactions to a work, not the work. It also shows humans are slow learners. We wipe sometihng "evil" from the face of the earth only to discover "er, I think we needed that.."