Five years ago my friend (let's call her Jane) discovered her husband (let's call him Steve) was having an affair with a colleague.

For a few weeks, their marriage teetered on the brink. Jane was shocked at Steve's "betrayal", uncertain whether she could ever trust him again. Everything – including the future for the couple's two children – hung in the balance.

This week I met Jane for lunch... and what a difference five years has made. She's happy, fulfilled, forging ahead with an important work project.

She and Steve have just had a holiday à deux and their marriage seems unshakeable.

But Steve still sometimes sees his other lover. And what's more, Jane now has an extramarital love interest, too. "We've got a new arrangement," is how Jane explains it. "Sometimes I think it's complicated – at other times, it seems ridiculously easy. Basically, we sat down and worked out that we're really happy with what we've got together – a lovely home, three gorgeous kids, fulfilling jobs. But we've been honest about the fact that we sometimes need a bit more."

Rewriting the rules around marriage, as Steve and Jane have done, is catching on, and there's a spate of new books and movies out to prove it: books that aim to unpick why we can't be more imaginative in the ways we live out our long-term relationships and films such as Hope Springs that seek to remind us that a long-term relationship doesn't have to dissolve into a stale and lonely old age.

Psychologist and relationship therapist Meg Barker is the author of Rewriting the Rules (Routledge), out this month. She says there are several differences in today's long-term committed relationships that underpin the need for change. "Number one is that people are living to be a lot older – so a long-term relationship is a much longer deal," she says. "And another thing that's changed is expectations: we require so much more from a relationship than people did in the past.

"There's been a huge growth in the recent past of this idea that you need one perfect relationship; that you will get everything from it – romance and children and financial stability and friendship and a great sex life. No other generation had such huge hopes invested in just one relationship and it is an enormous ask."

An enormous ask that, more and more, is prompting people to wonder if the time has come to dismantle the scaffolding that holds marriage together and to look at whether it couldn't be constructed in another way. Because perhaps the over-expectations we've come to invest in marriage have made the scaffolding too shaky: maybe the time has inevitably come to realise that, as a society, we've been piling too much weight onto just one frame.

That's certainly how social scientist Catherine Hakim sees it: and her take is that it's Anglo-Saxons who are worst at loading the weights onto marriage and then watching as it wobbles under the strain. No surprise, she argues in her new book The New Rules: Internet Dating, Playfairs and Erotic Power (Gibson Square Books), that it's Britons and Americans who have the highest divorce rates on the planet – because these nations are also the ones whose citizens have the highest (and, she would argue, the most unrealistic) expectations of the institution itself.

More than 90 per cent of Americans and 80 per cent of Britons condemn extramarital affairs as wrong, compared with just two in five people in Italy and France.

And guess what, says Hakim: in Italy and France, divorce is far less common. "There is no assumption [in these countries] that spouses must fulfil all of each other's needs, all of the time, exclusively," she explains.

Hakim's take is that affairs happen – and when they do, couples (especially in Britain and America) are using sledgehammers to crack nuts. That's what, she would say, a couple like Jane and Steve would have done if they'd ended their marriage five years ago. At root, their relationship is fine: not perfect (but who, and what, is?), but happy enough, and friendly enough, and even sexy enough, and certainly functional enough to make a safe home for their children to grow up in. How tragic it would have been, Hakim would say, if a couple like Jane and Steve had chosen to unravel all that in the midst of what was, all said and done, a difficult chapter in their relationship – but one which, with some straight talking and broad thinking, they were able to work through.

The French perspective on affairs is very different: the attitude there is more philosophical and more tolerant. "Affairs are not actively recommended, but they are not prohibited either," says Hakim in her book. Contrast the UK, where (despite the fact that affairs are very common), the language around them is loaded with negativity (think "cheating", "dishonesty", "love rat"). When Jane heard of Steve's extramarital relationship, she felt "betrayed": but why, exactly, did it have to be a betrayal? Relationship psychotherapist Paula Hall of Relate agrees with much of the logic of Hakim and Barker. Her line is that if anyone thinks there's a safe place in a marriage, they're kidding themselves: marriage, like everything else in life, is risky.

"Monogamy has its risks – boredom being the main one – and an open marriage has its risks, too, in the form of jealousy, feelings of rejection and so on," she says.

"But there are real differences in the landscape of a marriage these days and they're about the internet and opportunities for meeting people as well as in how great our individual expectations are. So we are the generation that can move the boundaries here and look again at how to draw up what a marriage is about," she says.

It's even possible, she ventures, that monogamy has played out its usefulness to humankind. "Some experts argue that, from an evolutionary perspective, we simply don't need monogamy as much as human beings did in the past," she says.