Almost as soon as couples sit side by side on the sofa in my therapy room, they start to pour out the details of their latest rows, each other's bad habits, miserable childhoods and mutual recriminations. It's easy to get caught up in the drama acted out in front of me and to forget to ask about the one subject that they almost never volunteer: sex. And when I do bring up the state of their love life, they exchange embarrassed glances, as if asking each other's permission to speak.
"When we do, it's very nice," said Sarah, a 30-year-old management consultant and one of my patients. "We're very close and we enjoy cuddling and Sunday morning lie-ins," added Jake, her lawyer husband. "That's something you'd miss if we had children."
If I'd given them half a chance, they would have reverted to their arguments about when was the right time to start a family and fertility issues.
However, when I probed deeper, neither Sarah nor Jake could remember the last time they had had sex. It soon became clear they were in what sex therapists call a "sex-starved relationship" - which means less than 10 times a year. (Low sex is defined as only every other week.) Worse still, their love life had been dwindling over a long period - "probably since after we got married," admitted Sarah - and, although they had sought advice from a fertility clinic, they had waited five years before seeking help from me.
This couple are by no means unique. While our culture becomes more sexualised than ever before, we're less likely actually to be having sex, and we're certainly not talking about it - even to professionals like myself. I call it the silent epidemic.
After 25 years as a marital therapist, I was not surprised by last week's findings from the National Survey of Sexual Attitudes and Lifestyles, which canvassed 15,000 people and found that Britons aged 16 to 44 are having sex on average fewer than five times a month. When they asked the same question 10 years ago, it was just over six times. What's the problem?
Of course, some of it is obvious - taking our phones and tablets into the bedroom (and catching up on work emails or playing games), as well as porn entering the mainstream and becoming more acceptable (so it is easy to satisfy the biological need without being intimate with our partner).
However, a greater obstacle with some is the demand we place on ourselves as parents to be ever present and always at the top of our game. Something has to give.
"I'm afraid we didn't get a chance to do our sex homework," said Kate, 50, another of my patients. (I had given them a sensual touch exercise.) "We just didn't have the time."
"We did go to bed early last night especially, and then our daughter remembered that she needed to hand in her homework and Kate went off to type it up for her. Then, when she came back to bed, she was too tired," explained James, 53. Although their daughter was 17 - and more than capable of doing her own typing - Kate found it impossible to say no, even to a request at 10pm.
"Kate can say no to me," noted James, bitterly.
"But you want our daughter to do well," Kate snapped back. Once again, being a great mum and dad had trumped being a loving husband and wife.
Sometimes when I recommend putting a lock on the bedroom door, so parents have a private space and children can't just wander in, you'd think I'd suggested sending kids down the mines.
"But what if there's an emergency and they need us?" asked Carrie, a 38-year-old mum of three.
"They could knock and shout fire."
"But it only takes seconds for smoke to sweep through a house."
When we looked deeper at her resistance, I found a far bigger problem. Carrie was outsourcing responsibility to her partner for her sex life - and then being angry when he did not deliver. "I need him to turn me on and bring me out of mummy mode," she explained. "Otherwise I'm running over a list of what they need for school tomorrow and what I have for their packed lunches."
Meanwhile, her husband, Mike, also 38, was fed up with being the one to initiate sex. "It's me who always risks being rejected and repeatedly turned away. How does that make me feel about myself?"
Carrie didn't say anything, so Mike answered. "Not very good."
In effect, he had outsourced his self-esteem to Carrie. "If we do have sex - which is hardly ever - I'm walking around with a big smile on my face for the rest of the day and I'm even more effective at work too."
His wife tried, but failed, to console him: "But if Brad Pitt walked in to our bedroom naked, I wouldn't be interested ... "
Ultimately, we have to be responsible for getting in the right mood for sex ourselves - by learning to switch off from everyday concerns and not needing constant reassurance from partners.
Unfortunately, there are lots of myths about desire and sex that make this extremely hard. The most pernicious is that sex should be spontaneous. So when I suggest planning as one of the bridges from the everyday world of children, bills and chores into the sensual world of lovemaking, I meet plenty of resistance - even though we're happy to book concert, theatre or plane tickets and arrange to hook up with friends in advance rather than on the spur of the moment.
Partly it's a hangover from our Victorian past, where sex is OK as long as we're swept away on a wave of passion - and not fully responsible. However, it's also down to something else. Sheila, 58, said: "What if we plan but I'm not in the mood for sex?"
Sheila and Patrick had been together for more than 35 years, their children had grown up and Patrick's work was becoming less demanding. They should have been having the best sex of their marriage, but they had fallen into another trap that promotes low sex: all or nothing. They either had full intercourse or stayed over on their own side of the bed.
"I have to be sure that I'd be able to deliver," Patrick explained, "because I didn't want to start and not be able to finish, and actually I didn't think that Sheila was interested in sex."
"I thought he was too tired from work or depressed or having an affair and not interested in me," she replied.
I could think of nothing sadder than both wanting sex but not being able to talk about it for fear of upsetting the other. So I initiated a program to break "All or Nothing", where they would cuddle on the sofa while watching TV, giving permission for a cuddle to be "just" a cuddle. Therefore, when Sheila asked what to do if they planned an early night and they weren't in the mood, I turned the question back to her to answer.
"We could put on relaxing music and dance and cuddle, or have a hot bath together," she replied, "and who knows? We might get into the mood, or we could just enjoy being intimate together."
Interestingly, she had challenged another myth about sex: you either feel desire or you don't. In reality, desire takes time to build and it comes and goes (sensual touch is a great way both to enter into the zone and to bring it back if distracted).
Ultimately, what counts is the quality rather than the quantity of sex. So please don't feel that you have to hit a national target. However, if you'd like to improve your frequency, instigate this simple plan. Flirt with your partner during the day - send sexy texts, exchange private jokes and compliments - so you build a sexual connection. Co-ordinate bedtimes and body clocks, so you go to bed and get up at the same time, to maximise the possibility of sex and, finally, switch off electronic devices in the bedroom (and that includes the TV) so you don't undo all your good work.
Andrew G. Marshall is a marital therapist and author of I Love You But You Always Put Me Last: How to Childproof Your Marriage (Macmillan).
The Sunday Telegraph