Coward that I am, I'll post this remark anonymously. It's possible to have sex with a friend in a long-term relationship, acknowledging that you're having sex because you enjoy sex with each other, but you don't want to go into a committed relationship beyond fair-play in the sexual relationship. I've had a relationship like that for a number of years now. He is married, happily, and this isn't intended to compete with the wife and family. It works for us--and that's where we leave it. We're very careful about not broadcasting our relationship when we're around others in settings where either of us might be recognized.

Since neither of us is a fruitcake, as apparently may be the case of one partner in the scenario described by the erstwhile fornicator, this may not be a good example for him to try to emulate. But for others, I don't think it hurts to point out that if you set aside some of the Christian dogma, it's possible to have this kind of relationship and not suffer for it.