Traditional couples therapy focusses on the defense and enforcement of the monogamous pact, and tends to side firmly and explicitly with the faithful spouse. He or she is often referred to as “the injured party,” while the straying partner is labelled “the perpetrator.” The standard assumption is that an affair is a symptom either of marital dysfunction or of some pathology on the part of the perpetrator. (Sex addiction and fear of intimacy are the most common diagnoses, although lately a genetic predisposition to infidelity has been gaining traction.)

This approach, Perel believes, does little justice to the “multifaceted experience of infidelity.” It demonizes adulterers, without pausing to explore their motives. It focusses on the traumatic effects of affairs, without acknowledging their “generative” possibilities. “To look at straying simply in terms of its ravages is not only reductionistic but also unhelpful,” she writes. Affairs can be devastatingly painful for the ones betrayed, but they can also be invigorating for marriages. If couples could be persuaded to take a more sympathetic, less catastrophic view of infidelity, they would, she proposes, have a better chance of weathering its occasional occurrence. When people ask her if she is against or in favor of affairs, her standard response is “yes.”

Perel, who is Belgian-born but practices in New York, is much sought after for her sophisticated, European-flavored insights into love and desire, and she has made a specialty of challenging the puritanical orthodoxies of the American therapy industry. “Mating in Captivity” (2006), the book that brought her to public notice, was a sprightly disquisition on the anaphrodisiac effects of married life, in which she argued that the excessive value placed on communication and transparency in modern relationships tends to foster conjugal coziness at the expense of erotic vitality. Her suggestion that couples seeking to sustain their élan vital would do well to cultivate a little distance and mystery was not original, or particularly radical, but it inspired wariness and even hostility among some of her colleagues, who felt that she approached the solemn project of saving American marriages with insufficient reverence.

The new book, which expands on (and occasionally repeats) the ideas explored in the last, has met with similar objections. Perel has been accused of trivializing the scourge of infidelity and of promoting ideas that are fundamentally hostile to the institution of marriage. It’s difficult, however, to find any real evidence for these charges. Perel is more sanguine than others about the capacity of a marriage to withstand adulterous lapses, but her belief in coupledom—her commitment to the idea of commitment—is never in doubt. Insofar as she stresses the importance of flexibility, patience, and even stoicism in long-term relationships, her book bears a distinctly traditional message.

Perel takes a very stern line on what she sees as the excessive sense of entitlement that contemporary couples bring to their relationships. Their outsized expectations of what marriage can and should provide—perpetual excitement, comfort, sexual bliss, intellectual stimulus, and so on—together with their callow, “consumerist” approach to romantic choices, leave them ill-equipped to cope with the inevitable frustrations and longueurs of the long haul. They are too quick to look elsewhere the moment that their “needs aren’t being met,” and too ready to despair the moment that the promise of sexual loyalty is broken. Those who show willingness to forgive infidelity risk being chastised by friends and relatives for their lack of gumption. Women, Perel notes, are under particular pressure these days to leave cheating spouses as a mark of their feminist “self-respect.”

One reason, of course, that crises of infidelity attract such vampiric interest is that they lift the peacetime ban on judging other couples’ complex relations. For a moment, the wall of privacy around a marriage is breached and everyone gets to peer in and make assessments. The outrage and moral certainty expressed on such occasions can be comforting for the betrayed spouse, but they are largely “unhelpful,” according to Perel. In order to come to any adult reckoning with an affair, the betrayed must avoid wallowing too long in the warm bath of righteousness. For a period immediately following the revelation, a certain amount of wild rage and sanctimony is permissible, but after that the rigorous work of exploring the meaning and motives of an affair must begin.

The scrupulous evenhandedness of Perel’s approach is eminently reasonable in theory. She wants to redress a traditional bias against cheating spouses, to acknowledge “the point of view of both parties—what it did to one and what it meant to the other.” In practice, it must be said, her method seems to demand heroic levels of forbearance on the part of faithful spouses. They are asked not only to forgo the presumption of their own moral superiority but to consider and empathize with what has been meaningful, liberating, or joyous about their partners’ adulterous experiences. The affair that has caused them so much anguish may have been prompted by boredom or a longing for sexual variety, or it may have been a bid for existential “growth, exploration, and transformation.” (It’s hard to imagine anyone being gladdened by the news that his or her spouse’s adultery was an Odyssean quest for self-discovery.)

They are also asked to control their vengeful impulses, learning to “metabolize” their desire for vengeance “in a healthy manner.” (A healthy act of vengeance is making your spouse send a check to your favorite charity, not sewing shrimp into the hems of his or her trousers.) They must resist the desire to “know everything” and avoid demanding details about the physical acts involved in their partners’ betrayals. (They can ask “investigative questions” about feelings but not “detective questions” about hair color, sexual positions, or the size of genital organs.) Americans, Perel observes, are particularly inclined to believe that a process of forensic confession is a necessary forerunner to the restoration of trust, but “coming clean,” she argues, is often more destructive than it is salutary, and “honesty requires careful calibration.”