One year, in mid-June, I decided to get a diaphragm. This was my first truly adult gadget. By July, I had someone to use it with, and by August I had "cheated" on him. But it wasn't cheating because I promptly, without apology, told him. That summer, cheating seemed as out-of-date as chastity itself, something of concern to people even older than my parents. Intricate emotions that were starting to flower (or fester) in my not-very-secretive heart were so often diluted by the openness of that era.

At 21, I informed a 35-year-old lover that, "If you really like someone, you want them to be happy -- and if sleeping with someone else makes them happy, you'll accept it." "Nobody," Frank assured me, "likes anyone that much." Months later, he was proven right, when he introduced me to another of his girlfriends. Our brief, girlish affair struck him as a betrayal, despite the fact that we spent so much time discussing him.

For some people, cheating on a sex partner is an ethical headache. For others, it's a logistical problem. Infidelity may be a dirty secret, a literary goldmine, bad PR, great gossip -- take your pick. Bracing types regard cheating as a Darwinian mating ploy. Film critics, anthropologists, retired prostitutes, hard-working feminists, good conservatives and naughty leftists have divergent, yet similar, feelings about the world's oldest -- and stickiest -- relationship problem. Here are some immodest proposals, confessions, and a few accusations.