Porn elicits sex. Art may express desire, but porn creates it. Porn is an end in itself, but art seeks something beyond itself. So we are talking about the artists' intent. If I only want to turn you on, or turn myself on with what I am writing, it's porn. If I intend to explore the multitude of human experiences, clearly I would not exclude sex, but that would only be a part of what I was doing. Porn seeks to create a marketable product out of sex. A painting that has nudity is not considered porn if it is one-of-a-kind because it can only have one buyer. But if your intention is to create something that someone would take off to a secret place so he can masturbate, then it's not art. Which isn't to say it becomes porn when someone does that. Boris may draw an awesome Deeja Thoris, but even if someone takes his book into the bathroom, it isn't porn because he did it as a book cover, and because nobody wears clothing in Barsoom.