There are not an awful lot of things to look forward to as one ricochets towards middle age. The one silver lining is a renewed passion for sex. This you will know if you are a reader of sex surveys. The latest to spring to mind (there have been so very many) is the so-called Sex Census 2012.

While we are too bogged down in our 30s and 40s with child-rearing and worrying about the mortgage, the findings of this survey – jointly funded by Relate and "adult shop" chain Ann Summers, and published last month – suggest that when we hit our 50s we are absolutely raring to go. As for our sexual confidence, that supposedly peaks between the ages of 60 and 69.

Really? Who are these middle-aged chandelier-swingers? Where do they winkle them out from? And can they really be telling the truth?

In my not-so-meagre experience, if there is one thing we lie about once we get to a certain age, this is it. And if we are not exactly lying (as, quite clearly, the friend of mine is who keeps reiterating, loudly and long-sufferingly, how she still has to tell her husband "once a day is perfectly sufficient") then we are being, shall we say, economical with the truth.

Admit to having shoplifted. Admit to having a bit of a drink problem, or being bankrupt. But living in a sexless marriage? Never. Fidelity, monogamy, still sleeping with the same person after however many years – that is what we are all supposed to value most. To admit to anything else, is to admit to a societal failing so profound, so deep, so … almost spiritual, it's beyond the pale. It is something that cannot be acknowledged, maybe not even to each other, let alone to one's friends.

As one 44-year-old married woman, whom I interviewed for my own little informal survey, wrote in an email (anonymously, of course): "It's such a blissful scenario: separate beds, or even bedrooms, deep love for each other, friendship, shared parenting but NO PRESSURE TO HAVE SEX! The couples who are living it – you think they are going to let on? Men and women are far more prepared to talk about having extramarital affairs than about having a celibate marriage. There's far less shame in having loads of sex than there is having no sex at all."

Sex. Or rather the lack of it. It could be the nation's sordid secret. Perhaps even the world's. Instead of running a cover story on mothers breastfeeding six-year-olds, as Time Magazine did last month, celibate marriage is the taboo they should have addressed. How for millions of us supposedly happy, fulfilled, hand-holding couples, the spare room isn't the spare room anymore; how whenever we are cuddled up on the sofa and a torrid sex scene crops up on the TV, we both inwardly go "ewww".

Of course, there are those of you out there in your 50s, 60s and 70s still going like stink. Such as, for example, the nice lady I met at drinks the other evening who volunteered how she had her first orgasm at the age of 50, and has been merrily bonking ever since. But what of the swathes of who are very much not? Those of us who have been there and done that, with bells on, but at this point can think of nothing we would like to do together less, thanks all the same. Those of us who, yes, have sexual urges – many women find themselves suddenly becoming obsessed by sex during the perimenopause; it's like Custer's Last Stand – but find ways other than having a shabby, predictable affair to satisfy them. Why should we be the ones made to feel as though we are "living in sin", when we still hold hands, still basically respect each other, and still very much have stuff to say to each other over the dinner table?

Everyone – sigh – knows the drill: schedule it in; push through the repulsion/exhaustion/irritation barrier; engage in some roleplay; use it or lose it. But after one's broken the novelty shower-cap rule (I've got a cow and a frog one, you?), after one has spent 15-20 years cohabiting with the same perfectly delightful, perfectly attractive person, seeing them walk nude around the house about five million times, it can feel, frankly, a little … inappropriate. There's a reason, in other words, why kids retch at the idea of their parents doing it.

Oh dear. I might be out on a very long limb here. But there comes a stage in any relationship when the prospect of ordering your breakfast together the night before becomes far more the point of a saucy weekend minibreak than does the prospect of uninterrupted bonking. As Alain de Botton observes in his latest self-help manual, How to Think More About Sex, it was not until the 18th century that the idea of a love-based marriage took hold. The idea of sex being the cornerstone of a marriage came way, way later.

One couple I know, who are the same age as me, have children and live in the US, have not been having sex for a few years now. They seem to have arrived at a way of dealing with it. He goes to massage parlours for a "happy ending" every now and then, but would not dream of getting involved with anyone else. She, who is still as attractive as she was 30 years ago, doesn't ask. He doesn't tell, and they are fine. Ditto the 50-something friend with grownup children who, though happily married, has not had sex with her husband (or anyone else) in eight years. "People say sex cements a relationship," she says. "But at this stage of our lives together, I think it would actually impair it."

Then there are all those couples out there who, yes, can frequently be found at it – as it were – of an evening. Him downstairs in front of his flickering computer, her upstairs, er, reading her book. In other words, yes they are both having sex under the same roof, just not necessarily in the same room.

And what of the affectionate wife I know, now in her 50s, who cheerfully confides: "My husband? Hah! He hasn't seen a nipple for years."

The saucy sex survey is only partly to blame for this schism between myth and reality – the pretence that we are doing it every week, when in truth it is more like every six months (and even that might be an exaggeration). With a media that kids us into thinking ageing is negotiable, that so fetishises youth and beauty and so inaccurately represents what most of us walking down the street are actually like (not to mention our inability to talk about it without poking fun at it), is it any wonder? "As a nation we are sexually inhibited," offers Sue Newsome, a sex educator, tantra adviser and trainee psychosexual therapist. "We don't have frank, honest conversations about it with each other. It's not part of our national character. The upshot is that we create myths around our friends. Everyone is better than I am, everyone is having this wild sex life, while I am not. The grass is greener. Compound that with the images we are bombarded with via the media, where lots of young people are having fantastic sex and having orgasms exactly the same time as each other, and no wonder there is this rift between how we present ourselves and who we really are."

"Shampoo advert" sex. Yes. How shrivelling, how alienating that can be. As can all those American sitcoms where the cupcake-making mommy still wears a negligee to bed and is still hot for the daddy. Perhaps we need to take our cues from Scandinavian culture, where sex seems to be just part of your everyday routine, like brushing your teeth: a kind of Health & Efficiency thing, as opposed to this unbelievably momentous, fireworks-inducing event that nobody – not even Rachel Weisz and Daniel Craig, I'll wager – can live up to in real life. And even then, as the evolutionary anthropologist Helen Fisher, aka the "doyenne of desire", noted, romantic love – when you've got all that dopamine and PEA (phenylethylamine) coursing through your veins – can only physically last for a few years.

A married, male friend in the publishing business – 39 with one child and another on the way – still enjoys sex with his (extremely beautiful) wife, but is already aware of it becoming just another thing on the "to-do" list.

"The whole reason why it is so exciting in the first throes is because you can both objectify each other – that's really the whole fun of it, each of you both being able to treat each other as a piece of meat." He adds how the one-stop-shop pressure for a partner to be everything – a best friend, a fantastic lover, a fruitful provider and so forth – hardly helps. "How can you be that Milk Tray man or that Bond Girl you were when you first met for ever? It's finite. You can't fulfil every role." Right now, in the Venn diagram of sociosexual dynamics, he is in the most favourable position of all, where all three circles of fanciability, love and parenthood overlap (for me it was probably soon after the birth of my first child), but he can perfectly foresee a time when sex will dwindle to nothing. "Shared history is lovely and cosy, but ultimately it is not terribly sexy."

Or as Anthony Robbins, the motivational expert put it, in a slightly different way: "It is in the realm of uncertainty that your passion is found."

"There's a huge societal pressure at all ages to be more sexual," says Dr Petra Boynton, a psychologist specialising in sex research, "and that doesn't mean just having sex. It means having sex that is always exciting, different and novel. That, in a sense is why these surveys are so pernicious, because they are using image to stack up a myth, a myth which is in turn used to sell an aspiration about achievement and performance that one is never going to measure up to. This idea that we should be using those 18-30 years as a benchmark, that we should be scheduling date nights and going on sexy minibreaks to France; that if you aren't bonking like rabbits some terrible thing will befall your relationship."

So, then, can it be agreed, that not having sex after a certain point is normal?

"I wouldn't say it's 'normal', but then I hate that word," offers Guardian sex columnist Pamela Stephenson Connolly. "Normative, certainly. Of course, it's ubiquitous, and is a couple's right to choose that if it works for them. Not having sex is often a painful secret, though, because people always imagine there's something wrong with them (after all, since no one wants to confess, many couples think their friends are all having more sex than them).

"Many people go through a period of mourning for their lost sex life," she continues. "Even if they don't want it any more. Lots of things can happen psychologically. It's not the same for everyone, but I have come across many happy, sexless marriages."

"If you are having a lot of sex, and you are enjoying it, obviously I'm not going to talk you out of it," says Boynton, "but in this environment where we vet or measure our relationships by the amount of sex we are having, I think that is disingenuous for people who have lots of other ways to express intimacy. There are a number of things which connect people, but we are constantly spun this line that the glue to a relationship is sex, and without it one's relationship will fall apart, and I think there are a lot of commercial reasons why that message is put out. That's not just insulting, it's pernicious."

A post-sex state, then? A world where it's OK for marrieds to have separate beds, just like they did 50 years ago, and if they still sleep in the same one, do so in the manner of Morecambe & Wise? Is it such a sentence? Or might it not give us the freedom to fixate on something else, something we couldn't fixate on when we were younger because we were too busy fixating on you-know-what?

"We might be better off if we didn't have a sex drive," De Botton writes in his conclusion, "for most of our lives, it causes us nothing but trouble and distress." Gardening, then? It's not so bad after all.

Twitter @shithomemaker