The Depths of Desire by Joe Webster

Bill thought he had the perfect crime, a scheme which would satisfy both his glands and his wallet. Betty went for the idea. She didn't mind being raped by her husband, especially when they both stood to make a bundle on the deal. But the reward for the fake rape came in a form neither expected. A quirk of fate made them prisoners in a perverted place where homosexuality was the simplest of sins and where both discovered new depths of sensual desire.