Pornucopia — Piers Anthony

Prior Gross, the hero and sex object of this wild adventure, thinks his fantasies have all come true when a beautiful young woman seduces him on a public beach. She turns out to be a succubus, beginning his initiation into a realm populated by demons that are not merely horned, but horny. He encounters a perverse cast of characters that includes a satyr, a vampire, and a pair of luscious sisters, one of whom tricks him out of his manhood. So Prior Gross sets out on a perverse odyssey, taking him to a distant planet where he discovers the key to the return of his property and, ultimately, the origin of the universe itself.