When couples have been together for a long time what happens to the sex? Inside, readers provide some surprising answers, while Stuart Jeffries kicks off our special sex issue by asking if conventional coupledom inevitably means the end of passion

There are about 4,000 mammal species on Earth, but only a few dozen form lifelong monogamous pair bonds. The bonobo chimpanzees of Congo, for instance, eschew monogamy because they use sex as a social activity to develop and maintain bonds with male and female chimps. And monogamy is hardly the norm for humans. In his jaunty paper Alternative Family Lifestyles Revisited, or Whatever Happened To Swingers, Group Marriages And Communes?, family relationships professor Roger Rubin reports that only 43 of 238 societies across the world are monogamous. Many Toda women in southern India marry several brothers. Abisi women in Nigeria can marry three men on the same day. In rural Turkey, a man can marry more than one wife and each one takes on a different role. Even in the west, non-monogamy is actually the norm. Which is quite a surprise, given the psychosexual stranglehold the seventh commandment (you remember, the one about not committing adultery) has on Judaeo-Christian cultures. But it is the norm that dare not speak its name. In the US, 60% of men and 50% of women reported having extra-marital affairs. It takes the form, as Meg Barker, relationship counsellor, sex therapist and senior lecturer in psychology at the Open University, puts it "of secret, hidden infidelities rather than something that is openly known about by all involved".

That's to say, polyamory is all around, but socially inadmissible. "It is interesting," writes Barker in her new book Rewriting the Rules: An Integrative Guide to Love, Sex and Relationships, "that we readily accept someone loving more than one child, sibling or friend without their love for one of them diluting the love for others, but when it comes to romantic or sexual love most people cannot accept it happening more than once at a time."

She isn't suggesting that we junk monogamy, rather that we realise that long-term monogamous relationships as currently configured aren't so much fulfilments of love's young dream as disasters waiting to happen. In such circumstances, mere monogamy surely cannot bear so much weight.

Should we adjust our parameters? Should we pursue what relationship counsellors call the poly grail? Does sex matter to the health of a long-term relationship? Is it OK to give it up?

"We increasingly look for lots of different things in one place – namely the monogamous relationship," says Barker. Why? "Because we have become more and more atomised, work has become more precarious, community bonds have weakened and there has been a decline in religion, so we hope to get everything from one other person."

But that's surely impossible. In her book Mating in Captivity: Reconciling the Erotic and the Domestic, the therapist Esther Perel distinguishes between warm and hot relationships. The former involves absolute candour, togetherness, equality and, quite possibly, devising a mutually satisfying rota for picking up the kids from school and cleaning the toilet. The latter involves non-politically correct power plays and, if the book jacket is anything to go by, transgressive shoe fetishism as part of a sustainable sex life. Can one relationship be hot and warm at the same time? It seems, to put it mildly, unlikely. Does good intimacy make for hot sex? asks Perel. Again, unlikely: they don't sound like different rules, but different sports.

Conjugal felicity didn't used to be so conflicted, argues Alain de Botton in his new book How to Think More About Sex. Before the bourgeoisie introduced the idea of love-based marriage in the 18th century, he argues: "Couples got married because they had both reached the proper age, found they could stand the sight of each other, were keen not to offend both sets of parents and their neighbours, had a few assets to protect and wished to raise a family."

The new love-based conception of conjugal felicity, involving being physically aroused by the other's appearance, wanting to read poetry to each other by moonlight and yearning for two souls to fuse into one, changed all that.

Later, increased sexual expectations necessitated that the physical arousal and great sex you had at the start of your relationship be continued over years of your monogamous relationship – even though, frankly, most nights you'd rather watch The Great British Bake Off in old undies than tear off your partner's lingerie with your teeth.

Such expectations explain why you've got The Position Sex Bible: More Positions Than You Could Possibly Imagine Trying by Randi Foxx (possibly not a real name) unread on the shelves next to the unwatched DVD of Dr Sarah Brewer's Secrets of Sensational Sex.

And so it was that monogamy became made up of two equal parts – one involving endlessly deferred good intentions, the other nostalgia for When It Was Better. If it ever was.

De Botton applauds monogamy's unsung heroes, writing: "That a couple should be willing to watch their lives go by from within the cage of marriage, without acting on outside sexual impulses, is a miracle of civilisation and kindness for which both ought to feel grateful every day. Spouses who remain faithful to each other should recognise the scale of the sacrifice they are making for their love and for their children, and should feel proud of their valour."

Of course, not all monogamous couples have kids, neither are they all middle-aged, middle-class or heterosexual: but all of them, De Botton argues, deserve medals.

That said, De Botton also counsels that extra-marital affairs may be necessary. It's a thought shared by other anatomisers of that modern malaise, monogamy.

Former London School of Economics sociologist Catherine Hakim argues the following in her new book, The New Rules: Internet Dating, Playfairs and Erotic Power: "The fact that we eat most meals at home with spouses and partners does not preclude eating out in restaurants to sample different cuisines and ambiences, with friends or colleagues.

"Anyone rejecting a fresh approach to marriage and adultery, with a new set of rules to go with it, fails to recognise the benefits of a revitalised sex life outside the home."

If you're a 45-year-old woman or a 55-year-old man, you should probably stop reading this article immediately. Now is the peak time for you to have an affair. You should be on the pull for the sake of your marriage. Or whatever it is you call your relationship.

Hakim cites two economists who estimate that increasing the frequency of sexual intercourse from once a month to at least once a week was equivalent to £32,000 a year in happiness. David Blanchflower and Andrew Oswald also estimated that a lasting marriage provided the equivalent of £64,000 a year. "If you add the two together, an affair providing lots of sex and an enduring marriage, that's a recipe for a lot of happiness," Hakim concludes.

But this Panglossian summation of sexual happiness will only work if you keep schtum about your transgression. "I am happily married, and I would hope that if my partner had an affair he would be so discreet about it that I wouldn't notice anyway," Hakim told Jane Garvey on BBC Radio 4's Woman's Hour.

So Hakim does not recommend open relationships. Indeed, she is dubious about them. "All the literature I have read suggests they are imposed by men on women, or by promiscuous men on their gay partners."

Instead, Hakim tells me that if you're going to have an affair, you must play by French rules. "First and foremost, they must remain hidden at all times and never be visible enough to embarrass the spouse. Second, you never do it with someone in your own 'backyard' – neighbours, friends, work colleagues etc – where the risk of exposure is greatest."

But surely there are other risks of exposure? What if sleeping Mr Hakim lustfully groans the name of his lover in the marital bed, while Mrs Hakim sits bolt upright, eyeing him narrowly? At least a £64,000 reduction in happiness, is my guess.

Hakim's more serious point is that sexless, celibate relationships are unsustainable without some kind of sexual outlet. Across the Channel, sensible continentals realise that the answer to this condundrum is furtive infidelity. This is the main reason behind the sudden expansion of internet-dating websites that focus on married people seeking affairs.

"Only two fifths of Italians say affairs are completely unacceptable. One quarter of Spaniards do not regard sexual fidelity as important. The majority of the French – two thirds of men and half of women – believe that sexual attraction inevitably leads to intimacy. The incidence of affairs is informed by such tolerant attitudes."

Meg Barker, for one, is sceptical of the deceit such tolerance entails. "Why is deceit taken to be a good thing? The answer is to communicate. Today there are things like hook-up culture, friends with benefits, relationships that are monogam-ish, lots of different polyamorous possibilities. These kinds of things are up for negotiation."

What Hakim does, in effect, is uphold one of the bad old rules of monogamy that Barker seeks to junk, namely that the rules should not be explicitly discussed or negotiated.

Barker, by contrast, finds in monogamy's very indeterminate rules a space for confusion about what is permissible within a relationship. "One person may think it's all right to stay friends with an ex-partner. Another may think it's all right to flirt with or have sex with another person. Another may think it's OK to look at porn. What's important is communicating so you know what the other expects."

How important is sex in a long-term relationship? Barker says many of the couples who come to her seeking sex therapy expect that she will teach them how to have the great sex they had at the start of their relationship or have never previously enjoyed. "Sex is our whole idea of the barometer of a relationship's healthiness. So sex becomes this imperative. It needn't be. Sex is often portrayed as though, because you've had sex, your sex partner will know how you're feeling and respond perfectly to every situation in which you find yourselves."

This assumption that sex is the cause of and solution to any relationship problem is widespread in popular culture. In the recent film Hope Springs, for example, Meryl Streep and Tommy Lee Jones play Kay and Arnold, a sixtysomething couple who approach a therapist (Steve Carrell) because Kay is concerned about the lack of intimacy and sex in their long-term monogamous relationship. "The therapist in Hope Springs seemed to assume that Kay and Arnold had to recapture their sexual relationship, rather than really exploring whether this was something that they wanted and, if so, why it was important, and the different possible ways of doing this," says Barker.

When Arnold loses his erection, Kay assumes this means he doesn't find her attractive. Later, when they have what Barker calls "penis-in-vagina intercourse", their problems are resolved. "Penis-in-vagina intercourse is represented as 'real', 'proper' sex, and sex is seen as requiring an erect penis and ending in ejaculation," says Barker. "There isn't, for example, the possibility of sex which is focused on Kay's pleasure or the possibility of Kay and Arnold enjoying less genitally focused forms of pleasure. Also, erections are equated with attraction when these things may, or may not, be related."

Quite so. Is she saying it's OK not to have sex in a long-term relationship? "For some couples that may work, but not others. One possibility I address in the book is making a 'yes, no, maybe' list of all the sexual and physical practices that they are aware of, and whether they are interested in them. That may help."

Barker counsels periods of solitude in order to work out what you want from a relationship – or if you want out. "It's easy not to think critically about what's happening. It helps to create space to reflect on what you want."

Sex may well not be the biggest problem in a long-term relationship. "One of the biggest problems in a relationship is that it can be founded on someone validating the other, completing you by enabling you. So you have this idea that one partner in a relationship is a rescuer, or a mentor of a sweet young thing. It's in Fifty Shades of Grey – the broken man I made better. Fixing somebody like that or fixing yourself like that is to treat a person as a thing, which is always a mistake. If you're in a relationship for a long time it's harder to sustain those roles."

Indeed, Barker finds that a lot of couples come to her for counselling when these roles have started to fray. "The challenge then is to remake the relationship without those roles."

Tricky – like rebuilding a boat at sea.

But not impossible. "Monogamy is not an easy option. There's always going to be a sacrifice because there is a struggle between freedom and belonging. And at the outset you don't really know how much of one you're prepared to sacrifice for the other – or if you're prepared to make any sacrifice at all."

Freud wrote about this in Civilisation and Its Discontents in 1929: civilisation, he thought, is a trade-off between security and freedom. We swing one way and then, disenchanted, the other. On and on we go, aiming for perfect equilibrium without achieving it. Monogamy is similar.

Barker recommends that we abandon the old rules of monogamy and embrace uncertainty, guiding our relationships by means of creative negotiation. That way relationships can be made better if not perfect.

This chimes with what the psychoanalyst and writer Adam Phillips writes in his book, Monogamy: "All prophets of the erotic life are false prophets because every couple has to invent sex for itself. They are not so much making love as making it up."