Figures released last week by Britain's Office for National Statistics suggested that the "seven-year itch" exists. The dissolution rate among same-sex couples who entered into civil partnerships when they were established eight years ago has now reached 20 per cent, mirroring the general divorce rate.
But the seven-year itch is not the only marker in a long marriage. What about the other phases on the way from the initial honeymoon period to lasting contentment?
Here, we reveal the Seven Stages of Marriage...
What it is to be young and in love! The wedding was the best day of both of their lives. The bride looked radiant. The groom remembered to thank everyone in his speech. The champagne flowed, and when they went on honeymoon, they barely left their bedroom for the entire fortnight.
When they stagger exhausted off the plane, they move into their first marital home and tweet pictures of themselves standing on top of ladders with paintbrushes in their hands. They go to dinner with other smug married couples, and everyone agrees that although they're tremendously grown-up, they don't feel remotely old. They go clubbing all night and love the fact that they don't even have to think about picking someone up.
And they still have sex. Oh boy, do they have sex. She looks great in her barely visible Agent Provocateur knickers. His six-pack is still in shape. They're fecund and fertile and... uh-oh, is that a blue line on the pregnancy test?
There's tired. There's shattered. And there's being a mother. She can't remember what a good night's sleep feels like. She's up breastfeeding at 3am, devoured by infant gums and seething with resentment, while her bone-idle husband snores like a fat, old hog and wakes up demanding breakfast.
His libido, meanwhile, has almost recovered from the grievous mistake of looking down just as the baby's head poked out. But all she cares about is the baby. He loves the baby, of course he does - but he'd also like his wife to pay him a bit of attention. Preferably while horizontal. But she can't. She's just too knackered.
So they settle into parenthood and all those years of creches and kindergartens, parents' evenings and Christmas pantos. Though she does occasional Pilates and he has the odd run, they're just not as trim as they used to be. They sit there on a Saturday night, watching another madwoman-packed Danish crime DVD and finishing off a bottle of shiraz. And then...
There are few more dangerous times in a marriage than the aftermath of a silver wedding.
She meets a man at the gym. And he's funny, and there's a glint in his eye as he flirts and flatters in a way her husband hasn't since before the kids - there are two of them now - were born. Meanwhile, hubby's getting to know the 24-year-old girl in marketing, who still has the fresh, dewy optimism of youth. Not to mention the firm, unsuckled breasts.
The seven-year itch, they used to call it, timing those years from the start of the marriage. But that was before couples moved in together long before the big day. So now that itch can strike before the last layer of wedding cake has grown stale. Except that it's not really an itch. It's more like a vivid, festering sore.
Suddenly they're plunged into that particular hell of furious arguments conducted in a whisper in the hope that the children won't hear. Accusations and counter-accusations, denials and rebuttals, lies and half-truths fly back and forth, while their love lies bleeding on the kitchen floor. From here, there are only two ways to go. And all too often one of them is...
Lawyers have been called. Everyone talks about conciliation and mediation, but it soon turns into a savage, take-no-prisoners war fought with Weapons of Marital Destruction.
He sits there with his solicitor, whose hourly pay rate is only marginally lower than Wayne Rooney's, and is aghast to discover that his wife is even more of a scheming, manipulative b---- than he thought. She, meanwhile, is talking to her equally overpriced brief, who is telling her, in thrillingly cruel detail, just how to strip that b------ of a husband of every single penny he's got and to give him so little access to his children they won't even call him "Daddy" any more.
They end up in court and spend so much money that they have to sell the house they've been fighting over. At the end of it, the lawyers go off to buy more Porsches and Louboutins and two shattered, impoverished, embittered former spouses are left, amid the wreckage of their lives, wondering how it all went so horribly wrong. Unless, that is, they can change their minds and somehow stay together...
The three key ingredients in any lasting marriage are kindness, forgiveness and a bloody-minded refusal to give up. Our couple hung on through the bad times and now they're reaping the rewards. Their jobs are going well. They're as cash-rich as they ever have been or (though they don't know this) ever will be. They bought a bigger house and it's finally gaining enough value to make the mortgage look just a little less scary.
Their kids are growing up and they're not entirely horrible. Every summer, the family piles into the Volvo 4x4 and heads off for a fortnight with friends who have children the same age. The grown-ups get pleasantly sozzled at lunch, knowing that the youngsters are taking care of themselves by the pool.
It turns out it's possible to have quite a bit of fun in one's forties. She's got herself into enviably trim shape. He's not yet entirely Clarkson-esque. Sex may not be quite as frequent as it was when they were young, but though they do say so themselves, they're now a lot better at it.
There are few more dangerous times in a marriage than the aftermath of a silver wedding. In theory, this is the moment when the kids fly the nest, leaving their fifty- or sixty-something parents still young and prosperous enough to set off on a fun-packed odyssey of cruises, golf and cookery courses and safaris. The truth, however, is very different.
Those late middle-age years now see married couples coping with adult offspring who still depend on the bank of mum and dad, and either don't want or can't afford to leave home. Meanwhile, the couple's own aged parents stagger on and on, incontinent, gaga and burning up every penny of their estates on ruinously expensive care.
As the stresses mount, so does the marital tension. Still dreaming of better things, she's bored by her increasingly dull, lifeless, sexless husband. He's wondering whether there isn't just enough time for one last shot at a sports car and a young second wife.
This is the way we all hoped it would be. He's bouncing a grandchild on his knee and stuffing it with Werther's Originals. She's in the kitchen making a lovely cake for tea. Outside, bees buzz between the lavender and the hollyhocks in the charming cottage garden.
After all the trials and tribulations of a married life, they have reached the mellow sunset of their years. Their grandchildren are so adorable, such fun to spoil... and such a pleasure to hand back to their parents at the end of the day. Life is quieter now and, yes, there are aches, pains, hip replacements and moments of forgetfulness. But, by and large, life is good. By staying together, they are better off, healthier and happier than they would have been apart. The passion might not burn as hotly as it did (though the embers have not entirely died out), but affection, companionship and mutual respect remain. Dylan Thomas was wrong. Gently is just the way to go into that good night.
Sunday Telegraph, London