Former British PM John Major, England soccer manager Sven-Goran Eriksson, any number of royals— it can seem as if no one believes in monogamy any longer. Barely a week goes by without some new celebrity rumour — recently it was Jude Law and Nicole Kidman. While Jude Law denied he'd been unfaithful, another celebrity admits to having an affair. And it's not just celebrities: four in 10 marriages end in divorce, with adultery the most commonly cited reason. A recent survey by the British relationship counselling service, Relate, suggested that despite stated intentions beforehand, two-thirds of couples stay together following the disclosure of an affair.

Clearly, there must be quite a lot of infidelity about. Janet Reibstein and Martin Richards, authors of Sexual Arrangements: Marriage and the Temptation of Infidelity (Scribner), estimate that 'between 50 and 75 per cent of men, and only a slightly smaller proportion of women, have had or are having affairs while married'. Research in the United States proposed lower figures: approximately 25 per cent of married men and 15 per cent of married women. But whatever the exact numbers (and since affairs are by their nature secret, we may never be sure) it's clear that long-term, committed relationships are under pressure, probably as never before. Is there any point, any longer, in expecting or hoping for monogamy? Whether there is, or not, most of us do. The National Survey on Sexual Attitudes and Lifestyles found that only one person in 50 believes extramarital sex to be not at all wrong, and four out of five think it is always or mostly wrong. The figures are the same for the youngest adults as for the oldest. Research on monogamy in the UK, by Dr Kaeren Harrison and Professor Graham Allan, reveals two views: that infidelity is morally wrong, and that the character of modern society makes affairs more likely. These two positions are bound to be difficult to reconcile.

Certainly, it would have been extraordinary for an Edwardian wife to talk in similar terms to Kim. Historically, women used to look for husbands who would be kind and good providers, while men chose wives who would be socially suitable and attentive mothers. But, throughout the past few decades, this team which shares work has turned into a couple which shares emotions. So now we face a conundrum: we expect more of love than ever before — thrilling sex, overpowering emotion, practical teamwork and a life in public as a social pairing — and rely on monogamy to prove all this commitment. Yet the more we load on to relationships, the easier it is for them to fall short, not least as we also expect them to last for ever.

Today we think of marriage — for which also read long-term, committed relationships — as the place in which we will find ourselves. As Ulrich Beck and Elisabeth Beck-Gernsheim suggest in their analysis of modern relationships, The Normal Chaos Of Love, (published in the UK by Polity Press) we look to relationships to give life meaning: "Some powerful force has pushed its way in and filled the gap where, according to previous generations, God, country, class, politics or family were supposed to hold sway. I am what matters: I, and You as my assistant; and if not you then some other You."

There is, however, an inherent contradiction in the idea of marriage as a rolling revelation of personal destiny. Individual growth and autonomy can easily be at odds with the kind of compromises required for a long, loving relationship. "I love my wife and I respect her," says Tom, 42. "But we have grown apart over the years. When my father died, I found I couldn't talk to her about the anxieties it induced in me — the feeling that time was running out, I suppose. So I have settled for intimacy in some areas but not others."

Segment the marriage

For Tom, as for Kim and Ian in their different ways, the solution is to segment the marriage. "I feel as though my emotional life and the vivid part of my sex life are with William," Kim says. "What's left is the practical stuff. Ian would probably say that exciting sex for him is outside the marriage but his emotional life is in it, along with the practical and social stuff. Men seem to be better at dividing up sex and emotions."

There is anecdotal evidence that men find it easier to think of sex as something apart. But it may simply be that women prefer to justify their sexual enthusiasms with a dose of love.

"About monogamy," says psychotherapist Adam Phillips, "We would like to think that the sexes are different. That one sex is more moralistic, more conventional, more daring, more secretive, more lustful and so on. We'd like a neat division of labour, a bit of reassuring biology, some huffy, inspiring religion; even some enthralling psychology. Anything just to get it off our hands."

This is not to say that all breaches of monogamy are the same. People commonly differentiate their affairs according to whether:

a. they are casual or serious,

b. how long they last,

c. how much time is spent together.

There are further relationship is retaliation for a past affair, whether the spouse would mind, and whether it threatens to break up the marriage. But even these don't offer us nice neat categories. Sara, a mother of two in her mid-thirties, had a brief affair with her tennis instructor and, two years later, has still not recovered. "I found the affair moving and lovely and I am constantly aware of the lack of intensity in my marriage, but I have no idea what to do about it."

Maggie, a 45-year-old illustrator, had a series of brief affairs during her 'uneasy' first marriage. "Then I met Richard and I couldn't control my feelings any more. I ended up getting divorced and we've been together for 10 years. I don't regret those other affairs, or the break-up of my first marriage, but now I am monogamous. I make it my business to know who Richard is having lunch with and to tease him gently. I never have dinner with another man, even for work, unless Richard comes too. In fact, we try really hard not to go out separately in the evenings at all. I don't mind. If I did, it wouldn't work. But I like being with him and I know if we were more casual our lives would veer off in different directions."

The monogamy myth

Maggie has what Reibstein and Richards call the '’marriage-is-for-everything' model. Peggy Vaughan, by contrast, has written a book and runs a website devoted to debunking what she calls The Monogamy Myth (published in the UK by Newmarket Press). "The effect of believing that most marriages or committed relationships are monogamous," she says, "is that if an affair happens, it's seen as a strictly personal failing of the people involved." Peggy's husband, James, started having affairs after 11 years of marriage, continuing for seven years before he confessed. Peggy is now convinced that secrecy is the "most significant support for affairs in our society". Friends rarely tell the spouse if they find out; co-workers gossip, but otherwise just watch; and if partners do discover the infidelity, they are unlikely to broadcast it. She says: "The person having the affair comes to depend on this co-operation."

It's hard to quarrel with this, but harder still to see how Peggy's solution — complete honesty — could work. She argues that "Both partners should recognise that attractions to others are likely, indeed inevitable, no matter how much they love each other. So they should engage in honest communication about the reality of the temptations and how to avoid the consequences of acting on those temptations."

Maggie believes honesty of this kind would undermine her monogamous marriage. "I don't want to know that Richard fancies some woman in the office, or keeps dreaming about the girl in the corner shop. Our marriage may possibly be built on a romantic fantasy, but plenty of things are built on fantasies— America, for instance. Richard and I tell each other that no one else could possibly make us happy, and if we didn't believe that the whole thing would crumble."

Reibstein and Richards's research included people who believed their affairs had nothing to do with their marriages, as well as some who felt that unfaithfulness was positively beneficial to their marriages. A much more common interpretation, and the usual starting point for marital therapists, is that the affair is indicative of something amiss in the marriage. "People will say there's nothing wrong; it came out of the blue," says Julia Cole of Relate, "But you find there have been triggers, signs, things swept under the carpet: bereavement, job changes, redundancies, work/life balance."

Jacqui, a 33-year-old fundraiser, began an affair a year ago, not long after going back to work after maternity leave. "It lasted three months. It wasn't terribly important and I can't really understand it, except in terms of feeling fed up with all my responsibilities - husband, baby, job, house. I felt I was the emotional backstop and no one was paying any attention to me. He was a way of reminding myself that I wasn't just this person who gave all the time." Beck and Beck-Gernsheim argue that the demands of today's labour market — that we be mobile, flexible, competitive and ambitious, prepared to move on whenever, regardless of social commitments — exert a contradictory pull. Released from tradition and driven to 'choose their own biographies', couples seek happiness from one another because other social bonds seem too tenuous or unreliable. But as women, particularly, 'come to see themselves as autonomous people, they are less ready to accept the solutions offered by previous generations - adjusting to your husband, sacrificing your own interests.’

We want monogamy more than ever. As Phillips says: 'Faith, hope, trust, morality; these are domestic matters now. Monogamy is our secular religion.' But too often, life pulls us in the opposite direction; and it may actually be that our obsession with marriage contains the seeds of its own destruction. "Monogamy is a huge investment in another person. In money terms, we wouldn't do it," says Julia Cole of Relate.

The truth is that monogamy has probably always been a way of life only for some. In the modern times, we have also added to our higher expectations of marriage a belief in the right to personal fulfilment. The combination is incendiary. Towards the end of Zadie Smith's The Autograph Man, (Random House) the hero's betrayed girlfriend yelps: "There will never be that moment, don't you get it? When you've had all the different people you want, when you're done, when you settle for me. People don't settle for people. They resolve to be with them. It takes faith. You draw a circle in the sand and you agree to stand in it and believe in it. It's faith, you idiot."

Like any other faith monogamy comes in different shades of intensity: the monogamy of the self-righteous, who regard their constancy as a moral triumph; the monogamy of the fearful, for whom, as Phillips says, staying put is a cure 'for the terrors of aliveness.' And the monogamy - the one we all secretly want - of people who don't find it that tough. "One is only truly monogamous," says Phillips, "When monogamy is no longer the point: that is, when one is in love."

(By arrangement with The Guardian)