Mark Oppenheimer has a thoughtful piece in the New York Times Magazine on whether marriages would be stronger if we were not so absolute about fidelity. He is wrestling with the views of marriage liberals like Stephanie Coontz, Judith Stacey, and, especially, Dan Savage.
Marriage liberals argue that societies have always harbored a variety of practices about marriage. Savage, writing from a gay man's perspective, promotes a "monogamish" approach, expecting that some relationships would be more stable if they openly accepted straying. Savage argues that is some people have sexual desires that cannot be satisfied by their partners, they need to change the marriage in order to get them satisfied some other way.
I think the marriage liberals are certainly right that in practice every society does have within it a variety of approaches to marriage, not all of which are strictly monogamous. Men especially, find strict fidelity tough going. Gay men and rich men are more prone to find sexual outlets in addition to their spouses. As a description of reality, it is hard to disagree with this picture.
However, what the marriage liberals usually fudge is a clear sense of proportion about who is monogamous - and even more so about who can be monogamous. Most marriages are monogamous - even in societies in which that is not strictly required. Most people, and especially most women, do want and hold to a strong standard of fidelity. I think Savage goes wrong when he treats sexual desires as needs that must be met, within the marriage or not.
Savage holds that the marriage is more important than strict fidelity, especially if the couple has children. I agree with that. I think adultery would cause searing pain to most married people, and they would be right to feel painful betrayal. Nonetheless, I honor those couples who have been able to work back from an affair to a functioning marriage again. I do not think it is possible in all cases, but I honor the moral heroism of those who try.