In 2008, US marriage therapist Mira Kirshenbaum outraged many with her book When Good People Have Affairs, which claimed the "right kind" of fling could "jolt people from their inertia". But the idea has gained ground.
The following year, French psychologist Maryse Vaillant published Men, Love, Fidelity, a book that claimed couples would be happier if they acknowledged many men loved their wives but still needed "breathing space", and that the "pact of fidelity was cultural not natural".
That is also the view of Gweneth Lee, a 45-year-old businesswoman who divides her time between her native US and Chelsea, London, and who's been a mistress to several married men, convinced she's doing them a favour.
"When my lovers were with me, they were much nicer to their wives, so their wives were nicer back to them. The marriage stayed together, no one was heartbroken and no one ended up poorer," she says.
It was the same need for an outlet that drove Lee - whose husband died of liver cancer when she was 31 - to seek an affair, after her next partner became impotent. Wanting sex but not wanting to leave him, she was urged by a friend in a similar situation to join Illicit Encounters. Through it she met a married accountant from Hertfordshire with five children.
"He was a lovely man whose wife had gone through the menopause and started drinking, since when there'd been no sex for eight years," Lee says. "After he started seeing me [during which time Lee's own relationship ended], he became so much more cheerful, had a lot more energy and was willing to help around the house. His wife noticed and as he became nicer, she drank less and decided she wanted to win him back."
The affair ended soon afterwards. "He said: 'My wife is my priority.' I didn't mind; it was the honourable thing to do," says Lee.
But according to Kirshenbaum, affairs can succeed only if they are kept secret; cheats can never alleviate guilt by confessing, nor must they view their lover as a potential second spouse. Research suggests that only about one in 10 affairs leads to a long-term relationship, of which only about 10 per cent become permanent.
So can this recipe for a happy marriage - using adultery to breathe life back into the relationship - really work? Relationship counsellor Paula Hall thinks not.
"Dishonesty causes considerably more damage to marriages than anything else," Hall says. "If one person is having an affair behind the other's back, it stops the pair of you dealing with problems, or either of you moving on to a new relationship where you could both be happy."
An open relationship, on the other hand, can work, she believes. "If your contract of fidelity to each other doesn't include sexual behaviour, then there's not a problem."
All of which makes the terrain of adultery something of a minefield. Straying, it seems, is rarely without consequences.
As Hall says, the important question to consider is why are you doing it? "If the answer is, 'Because I don't love you any more,' then it's likely it's going to be the end of the relationship; if not you can work at rescuing things."
What to do if you've caught them cheating - or been caught yourself
1. Do nothing
"Avoid kneejerk reactions; you'll be rushing into a decision you may have to live with for the rest of your life," warns Hall.
2. Work out the extent of the infidelity
"Was it just a quick cuddle at a party or has he had a mistress for over 20 years?" she asks. "The scale of the misdemeanour makes a huge difference to your reaction."
3. The guilty party must show remorse
"Without a sincere commitment to change, this isn't going to work."
4. Reflect
"Both sides need to ask what are their motivations for wanting to be in the relationship. It may be children, it may be finances, but there needs to be something about the quality of the relationship to keep you there."
5. Be honest
"There has to be a commitment to full disclosure and full accountability going forward. It's a cliche but it takes years to build trust and a second to fracture it."
