"Fulfill" is a bit of a weasel word, isn't it? Suppose one partner would like to make love every night. The other, less libidinous spouse is more a two-or-three time a week type. We might say that the first spouse is "unfulfilled," but that sounds like a really poor excuse for adultery.

If the lack of "fulfillment" amount to some deep incompatibility, a good question to ask first might be: have the partners in the marriage talked about what's not working? Can it be fixed? If the answer really seems to be no, then the next obvious question is whether the marriage is worth saving.

Life is complicated, of course and blanket generalizations don't do justice to the complexity of people's relationships. But the old question: "How would I feel if the tables were turned?" is always a good one to ask when we're trying to decide if we're acting rightly. It's not just an old bromide; it gets at something pretty deep in our notions of right and wrong.