"I have sex with my partner once a week, and we're both quite happy with that," says Robert, 33, a solicitor. "Sex is not the be-all and end-all of our lives - we're not that sad. We used to do it more when we were first going out, but frankly these days we're just too knackered. I have to confess that in front of my male friends I do tend to pretend that we do it more often; it's a real man thing to fancy slappers like the Spice Girls and get bawdy when you're together in a group of blokes over the secretaries or whoever. But knowing that I'm exaggerating myself means I tend to take other men's tales of lust with a pinch of salt."
But even once a week is a pretty respectable total, says Philip, 32, who works in the city. "Somehow we just don't get round to it very often," he says. "I honestly couldn't say exactly when we last made love, but it was some time in the last couple of weeks. We are perfectly happy, but we just don't feel the need to bonk like rabbits."
Libido is an elusive thing, controlled by sex hormones like testosterone and influenced by a completely random array of sights, smells and feels, depending on the individual. Libido has nothing to do with fertility (falling sperm counts are not connected with lower sex drive). It is possible to measure arousal on a human subject, using sensors that measure excitement, but for large- scale surveys, the only measure is quite simply asking people about their sexual appetite. But this may not always bring truthful answers, because today, a lack of interest in sex is regarded with the same alarm and horror as promiscuity was in the Victorian age.
Some women are finding that sex in their relationship is dwindling - and they don't like it. "We haven't had sex since our baby was born, more than a year ago," admits Marianne, 29. "I thought my husband would be really upset by it and get all frustrated and hard-done-by, but we seem to have got into a routine where neither of us really misses it. He certainly hasn't ever complained. I'm now starting to feel it's a bit odd and wondering if I ought to worry about whether he still fancies me."
"I was stunned that two former boyfriends of mine confessed that they had been in relationships where they hadn't had sex for a year," says Louise, 30. "That would drive me mad. I just don't see the point of a relationship without sex. I think men seem to like lots of sex at the beginning of a relationship and then tail off; women like a steady flow of sex."
"I couldn't believe that after we were married we got to the stage where we were only having sex once a fortnight or even every three weeks," recalls Sarah, 29. "I made quite a deliberate and concerted effort to get things going again and it does seem to have worked. But I just never thought I'd end up having to do any such thing. In fact, I found it incredibly upsetting."
Relate, the marriage guidance network, conducts a large-scale examination of their psychosexual counselling clients every ten years or so. The most recent was published last year and seven per cent of male clients were complaining of loss of libido. "That may not sound like a lot," says Julia Cole, counsellor, psychosexual therapist and Relate spokeswoman. "But in the detailed survey we carried out before this one, only half as many men were citing this."
The most frequent complaint was erection problems, diagnosed for one in four men. "Often, such problems are linked to loss of desire," says Cole. "And in the Eighties, the top problem was premature ejaculation; that has been overtaken by erection problems. We think that men are now feeling more able to talk about their sex problems and instead of saying they've got a physical problem that needs sorting out, they are saying openly that they have lost interest. Also, women today are not settling for a sex life that doesn't meet their needs, and some men might find that intimidating, which could lead to problems."
Sex is not even fashionable, particularly among the under-thirties. In its round-up this week of the influence of Ecstasy on young people, Time Out magazine observes that these days the point of clubbing is dancing. "Getting laid just became incidental."
Susan Quilliam, relationships psychologist, agony aunt and the author of Your Sexual Self, believes this is symptomatic of a wider social change. "Just as women are saying 'We're up for it', men are saying 'We've got a headache'," she says. "Sex as the driving force for a relationship is very much out of fashion. There is a real feeling of choice among young people; a boy is not now desperate to prove he's a man by having sex. Young people are saying, 'We're not like our parents, who rushed into sex because they couldn't have it; now we've got it, we are going to put it in perspective.'
"I think the push for men to want lots of sex was largely social, because it simply wasn't acceptable for women to make the first move. Now that has changed and sex drive is being more equally distributed. The onus is off men to make the first move, so they don't need to be as driven."
The proliferation of men's magazines packed with helpful advice on sex is another sign that prowess is a sore point, believes Mick Cooper, author of The MANual: The Complete Man's Guide To Life. "All those articles in magazines like Men's Health on how to be the perfect lover - the underlying message is that men are worried, they are struggling to achieve.
"There is enormous social pressure on men to appear virile, and often men will stop wanting sex but not explain to their girlfriend what's going on. They'll just make excuses, like being tired, or wanting to watch something late on telly. Their girlfriend will know that's not quite true and will think he doesn't fancy her. I think that's hardly ever true. And talking about it may mean a return of arousal. If a man is worried about his virility and explains that honestly, talking about it will reduce his anxiety and he will be more likely to be in the mood."
But sometimes Sod's law intervenes. "In the last house we were living in, I was very keen but my boyfriend wasn't," sighs Cathy, 32. "Now we have moved, for some reason it's the other way round. I think it's to do with the classic old notion that if someone really wants you, you're not that bothered, and if someone is cool with you, you're really keen."
It's tough being a sex god
'I'm not a sex god. The problem is kind of the image. As you get older, that image isn't as cute any more - not like when you're 18 and going out with a bunch of girls. When you're 40 and you do it, it's kind of sad.'
'I first made love when I was eight, but I can't recall with whom. I have always loved women - blonde, brunette but preferably foreign. The moment a woman is attractive, it has a sexual connotation. AIDS has changed everything, it's frightening. You can't be so casual any more.'
'At high school, I was known as a heart-breaker. I wouldn't say I'm great in bed. Heart-throbs are a dime a dozen. A magazine named me The Sexiest Man Alive. I think that was a cruel, cruel thing to do. Who wants to be a standard for comparison?'
(self-confessed sex addict)
'My virginity was a burden. I don't believe in monogamy. I'm attracted by the primitive way a woman smells.'