 This characterization needs some narrowing down if one wants to arrive at a definition that captures the extension of what we ordinarily think counts as erotic art. For one thing, an extensionally adequate definition should exclude scientific-behavioral studies or medical illustrations of sexual activity (which tend to be neither erotic nor artistic in nature). Furthermore, it’s not the case, simply because some viewers take a painting or a sculpture to be about sexual activity, that we are ipso facto dealing with an erotic representation.  Someone who is unfamiliar with the story of the Christian Bible might take a crucifixion painting to be a depiction of a sado-masochistic act, but that would be insufficient grounds to call, say, Tintoretto’s Crucifixion (1565) a work of erotic art.