Seth and his girlfriend of many years were already engaged when he discovered she had cheated on him. It was only once, with a co-worker, but the betrayal stung. “I had jealousy, insecurity, anger, fear,” he recalls. “It was really hard to talk about it.” He wondered whether his fiancée’s infidelity meant there was something fundamentally wrong with their otherwise loving relationship. He worried it was a sign that their marriage would be doomed. He also still felt guilty about an indiscretion of his own years earlier, when he’d had a one-night stand with an acquaintance. “I knew that what I had done meant nothing,” said Seth, a New York-based entrepreneur in his early 30s. “It felt like a bit of an adventure, and I went for it.” But anxiety about these dalliances gnawed at his conscience. How could he and his fiancée promise to be monogamous for a lifetime if they were already struggling to stay loyal to each other? Did their momentary lapses of judgment spell bigger problems for their union?

For help answering these questions, Seth and his partner went to Esther Perel, a Belgian-born psychotherapist who is renowned for her work with couples. Her two TED talks – about the challenge of maintaining passion in long-term relationships and the temptations of infidelity – have been viewed over 15m times. Her bestselling 2006 book “Mating in Captivity”, translated into 26 languages, skilfully examined our conflicting needs for domestic security and erotic novelty. Recently she has taken her work further, into more controversial terrain. Her forthcoming book “The State of Affairs”, expected in late 2017, addresses the thorny matter of why people stray and how we should handle it when they do. When Perel is not seeing clients in New York, she is travelling the world speaking to packed conferences and ideas festivals about the elusiveness of desire in otherwise contented relationships. After Seth saw Perel speak at one such conference, he sought her out for guidance with his fiancée.

“Esther helped us understand that perfection is not possible in relationships,” he explains to me. With Perel’s help, Seth and his fiancée have come to embrace a relationship they are calling “monogamish” – that is, they will aspire to be faithful to each other, but also tolerate the occasional fling. “It just never occurred to us that this is something we could strive for,” he says. “But why should everything we built be destroyed by a minor infidelity?”

This view may sound sensible, but it remains heretical. Attitudes towards sex and sexual morality have changed dramatically in the past few decades, with ever fewer Westerners clucking over such things as premarital sex or love between two men or two women, but infidelity is still seen as a nuclear no-go zone in relationships. In fact, studies show that even as we have become more permissive about most things involving either sex or marriage – ever ready to accept couples who marry late, divorce early, forgo children or choose not to marry at all – we have grown only more censorious of philanderers. In a survey of public attitudes in 40 countries from the Pew Research Centre, an American think-tank, infidelity was the issue that earned the most opprobrium around the world. A general survey of public views in America , conducted by the University of Chicago since 1972, has found that Americans are more likely to say extramarital sex is always wrong now than they were throughout the 1970s, 1980s and 1990s. Younger generations can usually be relied upon to push sexual morality in a more permissive direction, but infidelity is the one area where the young and old seem to agree. In this broadly tolerant age, when so many of us have come around to accepting love in all different shapes and sizes, adultery is the one indulgence that remains out of bounds.

“There is no subject that elicits more fear, gossip and fascination in the realm of couples than adultery,” says Perel. Back when divorce was a shameful prospect, couples grappling with an affair typically found a way to muddle through. Now, however, men and women are often made to feel ashamed if they try to move past a partner’s infidelity, instead of “kicking the dog to the kerb”. This view is particularly popular in America, Perel adds, where “cheating” tends to be seen in purely moral terms. Critics of Hillary Clinton, for example, have long seen her tolerance of her husband’s infidelities as a blot on her character, rather than as a sign that she prioritises their strengths together over his personal weaknesses. This is a problem, Perel explains, because we have never been more inclined to stray.

Reliable statistics on infidelity are hard to come by as there are few incentives for candour and definitions vary. Numbers of those in Western countries admitting to some sort of infidelity range from 30% to 75% of men and 20% to 68% of women. Now that more women enjoy financial independence and jobs outside the home, the gap between philandering men and women is narrowing swiftly. “There is not a single other taboo that is universally condemned and universally practised,” says Perel. Basically, cheating is something we don’t want and don’t like, but it is something we do and do often.

Nowhere is the prohibition against infidelity in the West more severe – and the consequences more dire – than in America. “People in the States are massively hypocritical,” says Perel. “They don’t cheat any less than the French. They just feel more guilty about it.” Perel argues that this is because Americans not only have more puritanical views of sex and deceit, but also because struggling with self-control is central to the national ethos. “Everything is exaggerated here, everything is world-famous, the portions are gigantic, it’s all about excess and control. In Belgium you don’t sit and eat a meal and talk about all the things you shouldn’t be eating because it’s bad for you. Being bad is a pleasure.”

Perel wants to change the way we think about infidelity. Instead of seeing it as a pathological and immoral impulse that invariably leaves trauma and destruction in its wake, she wants us to understand that extramarital yearnings are all too natural, and that affairs are terribly, perhaps even inevitably, human. “Monogamy may not be a part of human nature but transgression surely is,” she says. “And sometimes even happy people cheat.” If, like Seth, we want to build relationships that will last, then we may need to share his realism about what such a relationship might look like, and what kind of imperfections we are willing to tolerate. “It’s not that monogamy is impossible to pull off, but a lot of people don’t and many more won’t,” he says to me. “The whole point of this is to maintain a relationship that can exist in happiness for decades. Esther’s been instrumental in helping us figure this out.”

“Infidelity was always painful, but today it’s ‘traumatic’,” says Perel. “This notion that ‘my whole life is a lie, I don’t know anymore what to believe’, or that you apply PTSD to infidelity? That’s a completely recent construct.” Raised in the Francophone Jewish community in Antwerp, Perel speaks with the kind of lilting French accent that could make a shipping forecast sound alluring. Between sips of kale juice at the Soho Grand, a chic Manhattan hotel near her apartment, she is explaining to me why time has hardened our view of adultery.

“It’s because fidelity is the last thing left that defines a marriage,” she says. “You don’t need to wait to have sex, you don’t need kids. You don’t even need marriage anymore. The only thing that distinguishes it is that, after years of sexual nomadism, you suddenly say ‘I have finally found the one. You are so extraordinary that I am no longer looking for anything else. For you I promise to be suddenly exclusively monogamous’.” The only hitch, says Perel, is that sexual nomadism doesn’t prepare you for exclusivity. “It’s not as though you got it out of your system. Love and desire aren’t the same thing.”

Perel has a refreshing way of talking about sex. Particularly in America, where schools still tend to advocate abstinence and where talk of sex swiftly veers into either smut or sanctimony, her non-judgmental ease with pleasure and desire is rare. Her delivery is also well-served by the fact that, at 58, she is still arrestingly attractive, with misty blue eyes, flaxen hair, an easy smile and an unapologetic way of carrying herself. Dressed in a stylish outfit of flowing bronze silk, which sets off her late-summer tan, she sits with her legs wide and leans forward, her elbows resting on her thighs, her finger- and toe-nails painted the same blood red. “Esther is one of the sexiest human beings I’ve ever encountered,” says Lisa Thaler, a psychotherapist in New York who asked Perel to be her supervisor after hearing her speak. “The way she thinks, the way she inhabits her body, she’s captivating.” When Perel says things like “Good lovers are made, not born,” her seductive confidence makes her easy to believe. Unlike past sex therapists who have become famous, such as the grandmotherly Dr Ruth Westheimer, Perel seems like someone who not only understands sex, but also is very, very good at it.

Seekers of marital advice also like the fact that Perel is still married to her husband of over three decades, Jack Saul, an American psychotherapist and the director of the International Trauma Studies Programme at New York University, whom she met while they were both graduate students in Cambridge, Massachusetts. “My husband deals with pain; I deal with pleasure. They are intimately acquainted,” she writes in “Mating in Captivity”. Together they have two sons, both in their early 20s. But Perel typically deflects attention from her personal life, and is quick to say that she is not holding herself up as a model. “Longevity doesn’t make a relationship a success,” she tells me. “My family life and my choices happen to work for me, but my choices aren’t what I am selling to anyone else. There are just as many reasons why I could not be together with him as there are that I am.”

Such humility is unusual among peddlers of relationship advice, particularly in a country where such guidance tends towards the moralistic and where only the happily married seem allowed to dole it out. Yet Perel is eager to make it clear that she is not selling dogma, but rather commenting on the romantic conundrums of our age. “What works for one couple may not be what works for another couple,” she says. “I really don’t think it’s one size fits all.”

Most people – including many couples therapists, particularly in America – assume that if you stray outside the marriage, there must be something fundamentally wrong with the union itself. But Perel argues that our motivations for affairs are far more complicated than that. “In an age of consumerism, an age of entitlement, we are never meant to feel satisfied,” she says.

Past generations may have been able to settle for fairly good marriages and so-so sex. “The old guy was happy to have a women lend him her vessel; the whole thing took four minutes, about as long as it takes to boil an egg. A soft-boiled egg.” But we now live in a culture in which we feel we deserve to be happy, we are entitled to it. “Everyone wants desire these days,” she says. “What is desire? It’s to own the wanting. I want. That’s the essence of consumerism.” Awkwardly for marriage, we rarely desire what we already have.

This is not a new perception, as countless women’s magazine stories entitled “365 ways to bring passion back into your marriage” can attest. What’s interesting about Perel’s work is her nuanced view of the erotic. Infidelity, she believes, is rarely about sex, or even about the other person. Rather, it’s about recapturing “a feeling of aliveness with someone, of playfulness and curiosity, of selfishness” – that is, the very feelings that time and the mundane necessities of life tend to erode in marriage. When we are unfaithful, Perel explains, “it isn’t so much that we’re looking for another person, as much as we are looking for another self.”

Desiring people other than our partner is fundamentally, unsettlingly natural. Helen Fisher, a biological anthropologist, argues that adultery even makes evolutionary sense, as affairs allow males to spread their seed, and females to diversify their gene pool and collect a little extra help on the side. But what we once tolerated as an unfortunate fact of life, we now see as traumatic. This, Perel argues, is because we not only expect our carefully chosen soul mates magically to satisfy all of our needs, but also rely on them to anchor us in an otherwise rootless and existentially lonely world.

“Never before has the private domain been the central place where people have to find the answers to all of the important questions of life,” Perel tells me. “People used to have religion, people used to have a community, people used to live with three generations of their family. But today I want my sense of belonging, my sense of identity, my sense of all the big questions of life located in my relationship with my partner and my children.” If our partners have essentially become our bulwarks against the vicissitudes of modern life, then it makes sense that infidelity has become rather more destabilising than it once was.

Yet Americans have a uniquely narrow-minded take on infidelity, says Perel. “Most Europeans see it as an imperfection, and not something worth destroying your marriage over.” But Americans, who tend to see sex as corrupting and approach pleasure with scepticism, often view affairs in more binary terms. “Here there’s a persecutor and a victim, these are the only two options,” Perel says. “The language is criminal. I think that speaks volumes.”

Perel of great price Esther in her library

Perel’s parents were both the only members of their large Jewish families to survive the Holocaust. Her father, the only survivor of nine siblings, went through 14 Nazi concentration camps and ultimately saved 60 people by creating a black market with a friend in the kitchen of one camp. Her mother made it through nine camps, outlasting every member of her Chasidic family. “If they had done what they had been told they wouldn’t have been alive,” she says. “What’s right isn’t always what people tell you, and the rules are sometimes corrupt and cruel. Those stories came with mother’s milk.”

The story of Perel’s parents is essential for understanding her and her work, she says. Yet she recognised this herself only after she turned her attention to sexuality. Her parents, she explains, emerged from the camps wanting more than just to have survived; they wanted to make the most of every day. “I began to understand eroticism not from the sexual modern definition, but from the mystical definition, as in maintaining aliveness, an antidote to death.”

Couples therapists in America, who number more than 50,000, rarely talk about sex. Most assume that if they fix a couple’s emotional problems, good sex will follow. “Therapists are humans and sex is a topic a lot of humans are uneasy about, so it’s no surprise a lot of therapists are uneasy when it comes to talking about sex,” says Ian Kerner, a New York-based psychotherapist and sex counsellor. Because couples therapists “receive very little training about sexuality and sexual diversity, their social beliefs often end up intruding into their practice without them being aware of it,” adds David Ley, a New Mexico-based psychotherapist who offers sexuality training to mental-health therapists around the country. Sex therapists, on the other hand, mostly deal with the medicalised and pathologised kinks of sexual performance. So couples who wish to talk about their flagging sex life or the appeal of a non-monogamous – or monogamish – relationship often struggle to find a willing therapist. As for infidelity, the lion’s share of America’s therapeutic literature focuses on the needs of the harmed partner and condemns the philanderer.

Perel’s approach is different. Not only does she get her clients talking about sex, ever mindful of the relevance of sexual desire in relationships, but she also sees infidelity as a complicated business that often lacks a clear villain or victim. “Betrayal comes in many forms,” she says. “You can be the person who has steadfastly refused your partner for decades, but then he cheats on you and you’re the victim? The victim of the marriage is not always the victim of the affair.”

Instead of treating an affair like a traumatic wound one partner shamefully inflicts on the other, Perel gets people to talk about why they strayed. “Before I tell a person you have to stop, I want to know: What is it for you? How mesmerised are you? Who are you in your affair?” Rather than punish people for their selfishness, their shortcomings, their lack of self-control, Perel wants to know what made them do it, what they were looking for, and why they felt they needed to stray to find it. “The debate is that once you make it complicated you’re trying to be a moral relativist,” she says. “But working with infidelity is about working with the existential dilemmas that surround commitment and loyalty and fidelity and love.” Sometimes, she adds, if a couple can be guided to ask the right questions and listen for the answers, a crisis of infidelity can help them talk about sex and intimacy in a way that brings them closer together.

This approach has its detractors. “Infidelity is a violation. And when you do something that destroys the well-being of the other person, it’s not neutral, it’s not fair, it’s not love,” says Janis Abrahms Spring, a Connecticut-based psychologist and author of the bestseller “After the Affair”, one of the first books to label infidelity a psychological trauma. “The reason my book has been so successful is because it provided a language that captured the heart of the hurt party and made them feel less crazy and alone. For Esther or any therapist to in any way minimise that pain is to retraumatise the traumatised patient.”

Others criticise Perel for her view that loving couples might struggle with desire. Psychologists who promote the attachment theory of human relationships argue that our most fundamental need is to create secure bonds with others, and it is only when we feel secure that we achieve emotional and erotic satisfaction. “Exclusive, positive-bonded relationships are the opposite of ‘captivity’,” argues Sue Johnson, an Ottawa-based clinical psychologist and couples therapist. “And secure attachment really precludes active deception. To suggest that people in happy marriages seek affairs is all kind of a fabrication. People have affairs because they get lonely, because they can’t connect with their partner. They tend to be into thrill-seeking and not into long-term relationships.”

John Gottman , a well-known American psychologist and researcher on marriage and parenting, sent me an e-mail in which he condemned Perel for having “very little clinical sensitivity, so her intuitions about people are almost always way off the mark”. By way of example, he recalled a video Perel presented at a professional meeting in which she treats a couple after an affair. “She asked the hurt wife to empathise with her husband’s pride at his prowess at sexually satisfying his affair partner. ‘Go ahead,’ she told the wife, ‘validate what a great lover your cheating husband thinks he was toward the other woman.’ We thought this was not only misguided but unethical and abusive. So she’s dead wrong. Basically about everything she says.”

Perel is not alone in proposing that we are guided by often conflicting impulses; the work of psychologists such as Stephen Mitchell and David Schnarch has paved the way for her. Evolutionary anthropologists such as Fisher have also found that humans are quite capable of feeling a deep attachment to one partner, an intense romantic love for someone else and a desire for hot sex with quite a few others. “We don’t have one fundamental human need, we have many,” says Perel. Or as Kingsley Amis once said of his own libido: for 50 years it was like being chained to an idiot.

But Perel’s charisma has raised the profile of this approach. She has become a mentor to many in her profession. When we meet in her Fifth Avenue office, just above Manhattan’s Museum of Sex (remarkably enough), she has just finished addressing nine established therapists who have sought her out for guidance – her second monthly meeting with therapists that day. Afterwards she will hop on Skype to advise a group of psychologists based in Israel, Hebrew being one of the nine languages she speaks fluently.

“Esther is really defusing the ticking time bomb at the heart of so many of our long-term relationships,” says Dan Savage, an American pundit who coined the term “monogamish” and is the author of “Savage Love”, an internationally syndicated relationship and sex-advice column. “We define cheating as a relationship extinction-level event, and then we stand around with our thumbs in our butts wondering why marriages don’t last.” Perel’s aura, adds Savage, helps spread her message. “When I say maybe you shouldn’t have a heart attack and die if there’s one or two infidelities over the course of a 50-year marriage, I’m one of those gay people who can’t keep it in his pants. When she says it, she’s a nice married lady who has dedicated her life and a great deal of her work to marriage counselling and trying to save relationships. I’m in awe of her. I just think she’s a genius, and incredibly insightful.”

Does her approach work? The question is irresistible, but also unanswerable, because “work”, in this context, can mean any number of things. Some couples never get past an affair, says Perel. Infidelity can become “a black hole trapping both parties in an endless round of bitterness, revenge and self-pity”. Others use adultery to expedite the collapse of a failing relationship. But after years of following up with couples she has treated, Perel has found that the ones who continued to thrive were those who used an affair as a catalyst for change. Of course it is natural to react to a betrayal with interrogations, injunctions, and near-forensic searches of phone messages and credit-card statements, she warns, but such things never quite allay anxieties that a partner will cheat again. It is only when couples stop scavenging for the sordid details and instead ask more probing questions about the meaning of an affair that they can figure out whether their relationship is based merely on exclusivity or whether it is grounded in the rarity of their connection.

“Maybe you really work to build a lifelong relationship that strives for monogamy but doesn’t expect it, at least not perfectly,” says Seth. “Talking about these things can be very scary at first, but it’s a process of getting rid of neuroses and insecurities. An irony is that infidelity actually makes your relationship more stable. Your partner is thinking, ‘Oh my god what other relationship am I going to find where someone is this secure that I can wander occasionally and still come back.’ It becomes another reason why you stay together.”

Although Perel became an American citizen in 2013, she remains a perennial outsider – a Jew in Antwerp, a Belgian in Israel, where she went to university, a European in America. This distance, and her way with languages, lends some heft to her observations of universal urges and local idiosyncrasies. Marcelo Bronstein, a friend of Perel’s for over 20 years, recalls going to a Spanish bookstore in a small Chilean beach town some years ago and spotting a sign that read “Sorry, we are out of ‘Mating in Captivity’.” “I thought, what is it about this Belgian woman that she can speak to these people in Chile? It’s as if she sees the patterns of humanity across cultures.”

Perel’s status as a foreigner also seems to give her licence to say things that might be off limits to insiders. She can be amusingly merciless in her take on her fellow Americans, and the naive way we seem to think “there’s a solution to everything.” In France, she explains, “a smart book is a brilliant ramble. The smarter it is, the more unintelligible it is. Here the art is about simplifying things. Six steps, seven steps – God forbid you go above seven! But the dilemma of modern love is a complicated situation, it’s not five steps!”

It will certainly take time before Americans soften their view of infidelity. Seth admits that he rarely talks about his “monogamish” relationship, “because it’s so taboo”. Yet he says that when he has opened up about it, at least among more progressive friends, “it’s almost like we’re heroes, like we’re inspirations to people who are thinking the same thing or are curious about it.” The fact that he and his fiancée have a good relationship and “are not like some hippy, dippy couple out on the fringes” often reassures people, he adds. “People seem glad to know that it can be done.”

This makes sense. In a country with so little tolerance for human frailty, where the pursuit of perfection often yields more shame than satisfaction, Perel’s message offers some solace. Perfection, she says, is impossible in even the best relationships. “A great relationship”, Perel insists, “is an imperfect one.”

PORTRAIT FLORA HANITIJO