It was a little trippy last night at the 92d Street Y. Sitting with an old friend, Dan Savage, and a seventies icon, Erica Jong, talking about sex in front of a few hundred Upper East Side denizens is not something you do every day. I said the f-word first, I'm happy to say, and after that, it was all downhill. For me the interesting point came when Dan and I agreed that moderate hypocrisy - especially in marriages - is often the best policy. Momogamy is very hard for men, straight or gay, and if one partner falters occasionally (and I don't mean regularly), sometimes discretion is perfectly acceptable. You could see Jong bridle at the thought of such dishonesty. But I think the post-seventies generation - those of us who grew up while our parents were having a sexual revolution - both appreciate the gains for sexual and emotional freedom, while being a little more aware of their potential hazards. An acceptance of mild hypocrisy as essential social and marital glue is not a revolutionary statement. It's a post-revolutionary one. As is, I'd say, my generation as a whole.