Photo by Scala/Art Resource, NY.

The divine lovers Venus and Mars recline, while Cupid lingers in the background of this fresco from Pompeii, now in the National Archaeological Museum in Naples. The ancient Pompeians clearly loved erotic art, and they put it on the walls of all kinds of buildings—public baths, taverns, villas and brothels. (The painting of Venus and Mars shown here is chaste by Pompeiian standards.) Why all this erotica? In “How to Find a Brothel in Pompeii,” author Thomas McGinn explains one of the uses of erotic imagery.