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Even the church seems to be backing the idea. Last November, the Dallas priest Ed Young urged the married members of his congregation to embark on a “sexperiment”, committing to seven straight days of “congregational copulation”.

Three bestsellers flaunt this “just do it” approach. Charla Muller’s 365 Nights: A Memoir of Intimacy describes how she slept with her husband every night for a year to help re-establish a connection that, post-kids, was on the wane. Similarly, Douglas Brown’s memoir, Just Do It, is a shameless testament to his and his wife Annie’s sexual “marathon” — 101 days of solid sex. Both peddle the same message, that sex with your spouse — good, bad or mediocre — will work wonders.

Why do we need their advice? The reasons married couples give for flagging sex lives range from boredom (the author and sex therapist Dr Pam Spurr calls it the “two-year turn-off”) to exhaustion. Careers often come first, but the arrival of children on the scene is often the clincher. Eight out of 10 new mothers report lowered desire, according to Relate figures. One man, responding to a Times Online sex survey, described sex after the arrival of kids as: “quick, covert, like a military strike”.

So clearly we have a problem. But won’t “just doing it” simply mean going through the (rushed, unsatisfying) motions? Not if you believe Brown. And what’s the alternative? “If you wait for ‘chemistry’, you’ll have sex once in a while, if you’re lucky,” he says.

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The problem is that one half is generally more up for it than the other. “Desire discrepancy is one of the most vexed issues in the modern marriage,” says Arndt. As research for her forthcoming book, The Sex Diaries, she asked 98 couples to chart how they negotiated sex within their relationship. And — surprise — in her study, it’s the women who are saying “no”. “Men are terribly conscious they’re on the back foot,” says Arndt. “The wives carry the power of rejection.” By contrast, she heard “a howl of despair” from the men about a lifetime spent grovelling.

Arndt refers to the “damp wood” of female libido (“it can need a lot of firelighters”), but a couple’s relationship cannot hinge on this, she insists. The idea that you could enjoy sex without “wanting” it can be a revelation, but “you just have to put the canoe in the water and start paddling”.

At the end of the 101 days, Brown’s wife proclaimed the sexperiment a success; it literally “screwed us together”. But not everyone feels the same way. One woman lamented, “I did try ‘just doing it’, but began to resent that I was just doing it for John. I felt like I was just servicing him.”

It seems, as in so many aspects of marriage, that the answer lies in compromise. The needs of both people be recognised. “Sometimes that means making love to your spouse even when you’re not in the mood,” says Spurr. “You do it because you know it’ll make the person you love happy.”

Amy, 54, scheduled sex every three days with her husband to potentially marriage-saving success. “On the ‘off’ days, I could enjoy as much intimacy as I wanted, knowing it didn’t have to lead to sex,” she says. Most important, “once we get into it, I always experience orgasm, and I feel that it brings us closer”.