"Everybody's at it." My friend spoke with an odd, sorrowful authority. All around her, everywhere she looked, people who had reached a certain stage in their lives and their long-term relationships had, after a few moments' hesitation, joined the game. It surprised her, particularly when one day she woke to discover that she was at it, too.
There is probably no need to quote generalised, empirical evidence to confirm that there is a lot if infidelity about, although yet another survey of our private lives has just been published; a study of the sexual habits of 40 Western countries suggests that in cheating on one another, if nothing else, we lead the world. We are in the grip of an "adultery epidemic".
Such surveys seem to be published every week, but anyone with ears and eyes will know that both my friend and the report are on the right track. Those cheap, out-of-the-way Italian restaurants that specialise in catering for those having affairs are busier than ever. Hotels are doing a heaving business, couples eagerly arriving without luggage in the early evening and checking out later that night.
Those caught up in the adultery epidemic take differing approaches to the situation in which they find themselves. Some – sophisticated or amoral, depending on your point of view – are Continental in their attitude. Infidelity is now the norm, they believe. In an age where self-worth is the holy grail, frustration and erotic boredom are the enemy. The sharp, dangerous pleasures of desiring and being desired are the right of anyone who wants to be truly alive.
Unfortunately, the British have never quite got the knack of the dispassionate organisation of their sexual beings. They make a mess of infidelity. Their separate lives, rather than moving forward smoothly in parallel lines – the domestic and the erotic – become hopelessly entangled.
Something has gone wrong. We are busy being terrifically modern about sex and duty, and it is making most of us unhappy. The genuinely contented long-term relationship has become a wondrous, enviable exception to the general rule.
Perhaps, after the next election, our government might express its famous moral concern by looking at the state of marriage today. It might well conclude that the institution, as it now stands, was designed for another age, when people lived shorter lives, were less sexually restless. What once offered society a kind of cohesion is now often held together by guilt, financial insecurity and social pressure. Maybe, between domestic imprisonment and a policy of shambolic sexual laissez-faire, a third way can be found – a 10-year, renewable licence, perhaps, a sort of marriage MOT.
Oddly, one of the most eloquent arguments for change to appear recently has come from two people who are faithful to one another in their fashion. In the latest Spectator, a 53-year-old woman writing under the pseudonym of "Rachael Jones" writes of her feelings on discovering that her husband, to whom she has been married for 30 years, has taken to downloading sexual images from the internet.
Mrs Jones pronounces upon this luckless man with lengthy, venomous disgust. "He made some gloomy remark about losing his sexual confidence. 'So what?' I flashed back. 'What the hell does it matter? Why does it all have to be such a performance?' " It's a nasty insight into the way of modern marriage.
Is this where we've arrived? On the one hand, the adultery epidemic; on the other, Mr and Mrs Jones, trapped together in domestic hell, with him diddling away upstairs to teensluts.com while she sends poison-pen letters to The Spectator?
The moment has arrived when we should stop agonising about the symptoms – infidelity, unhappiness, boredom, porn addiction – and look more closely at the disease itself.