'Before I met you, I was a civilised woman." This could be the catchphrase for every wife who is having an affair. It's a snatch of dialogue, uttered by actress Emily Watson, who can be seen playing wife and mother Yvonne Carmichael in new BBC One drama Apple Tree Yard, starting tonight.
An adaptation of Louise Doughty's bestselling 2013 novel, written for TV by Bafta-winner Amanda Coe, Apple Tree Yard tells the story of geneticist Dr Carmichael, who embarks on a passionate extramarital affair with a mysterious stranger she meets at the Houses of Parliament.
The four-part series not only has the temerity to feature a woman having sex outside her marriage - but also to make that woman older. Coe, herself in her fifties, explains: "Apple Tree Yard is that rare thing: a perfectly executed page-turner that's also a gripping exploration of the difficult moral choices we face in adult relationships."
It's also timely as, so says the latest research, more and more women are seeking extramarital excitement. According to IllicitEncounters.com, the UK's leading dating website for married people, 2016 was the UK's biggest ever year for affairs. More than 100,000 new members joined the site, 55pc of them women. The National Opinion Research Centre in the United States found that the number of wives who admit to cheating has risen by 40 per cent, compared with 20 years ago. And a Canadian study by the University of Guelph says that a woman is 2.6 times more likely than a man to stray if she's unhappy in her marriage.
Meanwhile, a YouGov poll in 2015 found that one in five people in Britain admitted to having had a fling, with a further 13 per cent saying they had considered it. Women were more likely to cheat with a friend, while men favoured work colleagues or strangers.
Age also has a bearing. A poll of 1,600 people in 2008 by Saga Zone, a social networking site for over-fifties, found that older people were more likely to take a relaxed view on infidelity - for a variety of reasons: they were raised in more liberal times or know that, given the average two-year span of most affairs, a brief and exciting distraction needn't ruin a lifetime of togetherness.
Now, these changing attitudes are being scrutinised on our screens, with shows such as Apple Tree Yard and American drama The Affair, starring English actress Ruth Wilson, reflecting a reality that doesn't always end as badly as it once did in fiction.
As women increasingly find financial independence and confidence, they are realising, usually in their forties, that what's missing from their lives is passion. With the children grown up and their partner's testosterone levels dropping, many find their libido has reawoken - even if they don't feel as good naked as they did two decades earlier.
Watson (50) agrees. "This idea that female sexuality is the preserve of 22-year-olds is just absurd. If anything, your sexuality blossoms during your late forties and fifties. You know what you want, you've done marriage and children, you're settled, you've got the menopause coming up, and I honestly think there's this moment where a woman thinks to herself: 'It's my life now - and it's far from over'."
Far from being physically unpalatable, Watson and Ben Chaplin, who plays her paramour, Mark, show mature infidelity as hot and electric as it is in real life. "Ben and I decided that we were absolutely going to make those sex scenes look believable … instead of just fumbling about the way you do when you're younger. Sexuality in older women is a very powerful thing," says Watson.
Whether you're embarking on a full-blown affair or simply imagining what it might be like to kiss the handsome man who works in the local wine shop, the fact remains that extramarital sex, even in contemplation, is very much part of a married woman's menu.
Growing up in the 1970s and having had my own marriage ended by infidelity on both sides, I understand the initial excitement it brings but also, for me, the heartbreak, tears and turmoil that resulted in lawyers' letters and the forced sale of the familial home. Now single, I still don't see infidelity - my own or that of my friends - as cautionary. There are always mitigating circumstances of which we may never be aware.
Among my friends, who range in age from 30 to 60, it's not a surprise to find a sea of infidelity. It's something revealed to no one but our closest, most loyal pals. Sharing these details is a vital part of modern female friendship - and if anyone were to snitch, that confidence would be terminated without question.
In my circle, we don't discuss infidelity as right or wrong. We listen to our friends' stories and accept that any liaison will more than likely peter out in time. Even if it leads to a divorce or, sometimes, a double life, we support them, right or wrong.
So why do my married female friends cheat? Do they succumb when passion and desire surpass domesticity? If the lure of new sex is greater than the boringly familiar? Research suggests women look elsewhere because the quality of their marriages is corroding. A 2014 American study found that a lack of quality time, unresolved conflicts and inattention were reasons women allowed their eyes to wander (and sometimes their bodies to follow).
Opportunity is also greater than in previous generations. The internet makes it easier to find other interested parties. The immediacy and impersonal nature of the web turns infidelity into a kind of marital incision: if you want to cheat, you can do it quickly and tidily. It needn't leave a scar.
Cheating is also free. Well, almost. For some women there is guilt. As one friend put it: "The burden of cheating on my husband is mine to bear. That's what I get for doing it: I am guilty for the rest of my life." But she will never tell him. Nor will I. She and I made a vow to carry this fact with us to the grave. She didn't cheat to hurt him but because she wanted to feel alive again.
According to David Frederick, a professor of health psychology at Chapman University in California and lead researcher on a 2007 infidelity study, while unfaithful men "were looking for sexual variety and excitement", women were more likely to cheat out of emotional frustration and "fall in love with someone else or to look for reassurance that they were still desirable".
This is the thread running through these new infidelity TV dramas and it rings brutally true. Apple Tree Yard may be more alarming to those who thought married men were the brains and loins behind affairs. Infidelity was never their domain alone, of course. But finally, fiction is catching up: women can and do cheat - and return to their marriage without paying a moral cost.