In high school, I always regretted never having sex with my ex-girlfriend. At a recent class reunion, I reunited with her—she still had traces of her youthful charm, but had transformed into a seductive, mature married woman. "I don't want to regret anything anymore. I want to make love to my ex," I resolved. I invited her out to dinner, and as I listened to her talk about her marital struggles, our bond grew warmer by the minute. That night, we finally had sex for the first time. But it wasn't enough to satisfy me—by the next day, we were craving each other again. Our relationship was evolving into something irreversible.