Now, certainly cross-culturally there have been a lot of societies that have built into the expectations of marriage a certain amount of tolerance [of affairs], and some of them even for females as well as males. In the United States, open marriage based on equality is really an untried institution — although there have always been exceptional individuals who have been able to do it. I think we’re getting more of them now as people come to marriage with more sexual experience, with higher expectations of friendship and intimacy, and less confusion of sexual attraction with love. I’m dubious, though, that we’ll ever institutionalize it.

And why is that? What is the main resistance to it?

I think it’s because we’re not the kind of society that has lots of very close, tight-knit relationships with a sense of interdependence that exists across the life cycle. So, we put more of our commitment eggs in the basket of our significant other. As long as that occurs, allowing sexual dalliances, which sometimes become invested with big emotions, will threaten that one source of obligation and altruism. Because we put this emphasis on the dyadic bond, it is a fairly rare couple that can go into [an open marriage] absolutely confident that [these other relationships] will not turn into something more threatening and permanent.

Why does sex in particular seem to pose such a threat?

We’ve made sexual enjoyment so important to a love relationship — which is a good step forward from the old days when a woman was just expected to submit whether she liked it or not — but then you combine that with our tendency in the United States not to have strong relationships and feel serious obligations once the romantic relationship has ended. Then any sexual attraction poses a sort of existential threat: “Will this [affair] spill over and then take everything away from me?”

It seems like we use monogamy to protect against the possibility of our feelings, or our partner’s feelings, being redirected toward another person — in other words, falling in love with someone else.

Right, right — that is the paradox. To the extent that we do use monogamy as a guarantee of that, we may blow the occasional infidelity out of proportion. We end up saying, “Oh my god, this is a threat to the relationship,” when in fact it might not have meant anything.

It seems that in addition to personal emotional barriers, the issue of social acceptance is a huge one. How do social ties play into and support traditional marriage?

Every society loads up its relationships with certain expectations that make it very difficult for anyone to chart a different course. One of the arguments for same-sex marriage is the fact that our society invests marriage with this very unique set of expectations of fidelity — not only within the couple, but also in how other people treat the couple, that they don’t try to break the couple up. We don’t have the same expectations for cohabiting couples. Throughout history, one thing in America that made women stay in unfair and unhappy marriages was the social expectation that men had more prerogative than women to [stray]. If you had a good provider who didn’t flaunt his extramarital affairs, you were expected to put up with it.

When I was researching this new book that we’ve talked about, “A Strange Stirring,” I was astounded by the marital advice books from less than 50 years ago that say, “If your husband was unfaithful, ask yourself if you’ve kept yourself well-groomed enough. Ask yourself what you’re doing to drive him away.”

Since then we’ve developed new expectations of fairness, and equal disapproval of male and female adultery is probably at an all-time high. The other side of that expectation is that it becomes more difficult for a couple to violate that formula. It’s the reversal of the expectations that make women put up with an unhappy marriage; now it makes it really difficult for people to have a happy marriage that doesn’t conform to the new expectations.

Why, after seeing sex scandal after sex scandal, are we not more honest about the difficulty of sexual fidelity?

God, that is a really good question. If we knew the answer, life might be easier! There’s this sort of naive thinking that somewhere, somehow one public figure is going to prove to us that you can do [long-term monogamy], and then there’s the indignation and second-guessing that happens when their infidelity is exposed.

Has the cultural definition of romantic love changed much over time?

It is starting to change. This is one of the big challenges for us, which goes far beyond whether you can develop a monogamous or non-monogamous relationship. As I argued in my book “Marriage, a History,” the notion that romantic love ought to be a part of marriage is a late 18th century invention. Before that, it happened outside of marriage and it was considered a liability to base a marriage on romantic love. It was only later that people began to emphasize romance as the basis for marriage.

There was a conscious recognition of how destabilizing it is to put love as the basis of marriage — because then you have the right to not enter a loveless marriage, even if you’re pregnant, and you can demand the right to divorce. The way that society handled that for the first hundred years was to define romance and love as this mysterious attraction of opposites. It was based upon weighty gender stereotypes. The woman loved the man because he was strong and powerful and protective. The man loved the woman because she was different and sweet and virginal. She looked up to him and he got to show off to her all of the things he knew and could do, and she was supposed to not stand in front of his light. So our definition of romantic love was based on this idea of falling in love with these extreme stereotypes, which also I think interfered with real friendships.

It’s only in the last 30 or 40 years that [men and women] have begun to build lives that are equal enough that we are really finding out that this old definition of romantic love — the frisson of the other being a stranger to you — doesn’t work. That’s infatuation, that’s lust. Now romantic love is based upon friendship and honesty — and, to an extent, that really does change the ballgame. Some people may indeed be able to negotiate non-monogamous relationships, while others will go through non-monogamy and eventually settle down.