In America, infidelity is described in terms of perpetrators and victims, damages and cost. We are far more tolerant of divorce with all the dissolutions of the family structure than of transgression. Although our society has become more sexually open in many ways, when it comes to monogamy, even the most liberal minds can remain intransigent. When discussing infidelity, we use the language of moral condemnation. And it isn’t only the act that’s reprehensible; the actor, too, is judged by the strictest standards. Adultery becomes a moral failing as we move to a description of character flaws: liar, cheater, philanderer, womanizer, slut. In this view, understanding an act of infidelity as a simple transgression or meaningless fling, or a quest for aliveness is an impossibility.
An affair sometimes captures an existential conflict within us: We seek safety and predictability, qualities that propel us toward committed relationships, but we also thrive on novelty and diversity. Modern romance promises, among other things, that it’s possible to meet these two opposing sets of needs in one place. If the relationship is successful, in theory, there is no need to look for anything elsewhere. Therefore, if one strays, there must be something missing. I’m not convinced.
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The current view is that infidelity depletes intimacy and is a breach of trust and commitment, both emotional and sexual, that can never be fully recouped. Even the psychological literature focuses almost exclusively on the ravages of infidelity. I’d like to offer a view that challenges this premise and encompasses both growth and betrayal at the nexus of affairs.
Though affairs often result in deep emotional crisis, deception and betrayal are not the prime motivation. I suggest we look at infidelity in terms of growth, autonomy, and the desire to reconnect with lost parts of ourselves. Perhaps affairs are also an expression of yearning and loss.

Lots more here from Esther Perel on infidelity, all of it fascinating. Though I've never been married or had an affair, this passage had a ring of familiarity:

Sometimes, we seek the gaze of another not because we reject our partner, but because we are tired of ourselves. It isn’t our partner we aim to leave, rather the person we’ve become. Even more than the quest for a new lover we want a new self.

The pressure on the institution of marriage is higher than it's ever been because we now expect our spouses to provide so many different forms of fulfillment. In an interview with Slate, Perel notes:

What’s changed is, we expect a lot more from our relationships. We expect to be happy. We brought happiness down from the afterlife, first to be an option and then a mandate. So we don’t divorce—or have affairs—because we are unhappy but because we could be happier.

This reminded me of two of my recent posts about the new definition of marriage: marriage is now all-or-nothing, and hedonic marriage.