As a general principle all art is erotic, though there are many degrees of quality. But art is not made to turn anyone one. Arousal of the artist has to be turned into enlightenment for the viewer – anything less is a fail. But there’s no limit to how explicit art can be, as long as that principle holds. This is not a moral sentiment, it’s a pragmatic distinction.
Pornography is not bad, but art has to distinguish itself from pornography anyway, although in the end pornography might be less about excitement than about the monkey’s normal curiosity. The other pitfall is what is called in the erotica business “tastefulness.” Carolee Schneeman’s film Fuses is formally really great, kind of a masterpiece in fact, but not lacking a certain coyness at times. Sex has to be elemental, visceral, shameless and untainted by artiness in order to be art.