As a society, however, we are further from turning off the porn than we have ever been. Pornography is everywhere - it masquerades as "gentlemen's entertainment" in the form of clubs such as Spearmint Rhino, it infiltrates advertising and it will soon be available in our back pockets, thanks to a deal by adult entertainment giant Private Media Group to beam porn to UK mobile phones.

In its hardcore form, pornography is now accessed in the UK by an estimated 33% of all internet users. Since the British Board of Film Classification relaxed its guidelines in 2000, hardcore video pornography now makes up between 13% and 17% of censors' viewing, compared with just 1% three years ago, a rate of growth that is being cited as a causal factor in the recent bankruptcy of Penthouse, at one time the very apotheosis of porno chic but in recent years little more risqué than Loaded. In the US, with the pornography industry bringing in up to $15bn (£8.9bn) annually, people spend more on porn every year than they do on movie tickets and all the performing arts combined. Each year, in Los Angeles alone, more than 10,000 hardcore pornographic films are made, against an annual Hollywood average of just 400 movies.

Pornography is not only bigger business than ever before, it is also more acceptable, more fashionable, more of a statement of cool. From pieces "in praise of porn" in the normally sober Prospect magazine, to such programmes as Pornography: The Musical on Channel 4 last month, to Victoria Coren and Charlie Skelton's book, published last year, about making a porn film, to the news that Val Kilmer is to play the part of pornography actor John Holmes in a new mainstream movie, there is a widespread sense that anyone who suggests pornography might have any kind of adverse effect is laughably out of touch. Coren and Skelton, former Erotic Review film critics, focus on their flip comic narrative, scarcely troubling themselves with any deeper issues. "In all our years of watching porn," they write, in a rare moment of analysis that doesn't get developed any further, "we have never properly resolved what we think about how, why and whether it is degrading to women. We suspect that it might be. We suspect that pornography might be degrading to everybody."

With pornography, it seems as if the sheer scale of the phenomenon has, in time-honoured capitalist fashion, conferred its own respectability; as a result, serious analysis is hard to come by. Only occasionally, amid porn-disguised-as-documentary that distinguishes much of Channel 5's late-night output, is there broadcasting that gives any kind of insight. Channel 4's documentary Hardcore, shown two years ago, told the story of Felicity, a single mother from Essex who travelled to Los Angeles hoping to make a career in pornography. Arriving excited, and clear about what she would not do - anal sex, double-vaginal penetration - she ended up being coerced into playing a submissive role and agreeing to anal sex. Felicity - the vicissitudes of whose own troubled relationship with her father were mirrored by the cruelty of the men with whom she ended up working - eventually escaped back to the UK.

Hardcore offered a rare, unadorned look at the inside of the industry, as did Pornography: The Musical, albeit in a more surreal form, with actors interrupting sex to break into song. Yet what about the millions who consume pornography, the men - for they are, despite pornographers' claims about growing numbers of female fans, mostly men - who habitually use it? How are they affected? Is pornography, as most these days claim, a harmless masturbatory diversion? That episode of Friends, albeit with tongue in cheek, suggested a heavy diet of porn might encourage men inappropriately to expect sex. Is that true? And what about more profound effects? How does it affect relationships? Is it addictive? Does it encourage rape, paedophilia, sexual murder? Surely tough questions need to be asked.

First, though, some definitions. According to the Shorter Oxford Dictionary, the word "pornography" dates to 1864, when it described "the life, manners, etc of prostitutes or their patrons". More recently, it has come to signify material, in the words of Chambers, "intended to arouse sexual excitement". Its most common themes, however, are power and submission. By contrast, "erotica", which is pretty hard to find now, carries additional connotations of "amorousness" and is far less concerned with control and domination. No, it is pornography plain and simple, from teen magazines such as Front to venerable "wrist mags" such as Playboy, to the almost daily bombardment of teaser pornographic emails, that confronts all of us on a ceaseless basis.

The received wisdom, pushed hard by such mass-market magazines as Loaded and FHM, is that men derive a pretty uncomplicated enjoyment from pornography. That, certainly, is the argument put forward by such proponents as David Baddiel, AA Gill, who has directed his own pornographic film, and the musician Moby, who once said in an interview, "I like pornography - who doesn't? I don't really trust men who claim to not be interested in porn. We're biologically programmed to respond to the sight of people having sex." Danny Plunkett, then features editor of Loaded, takes an equally relaxed view: "We know that a lot of people enjoy it and take it with a pinch of salt. We certainly don't view it as dangerous."

But is it as simple as this? One of my best friends is a man for whom pornography has apparently never held even the slimmest interest. Moby may choose to distrust him, but his sex life otherwise has always seemed to me perfectly robust. He is, however, so much in the minority as to seem almost an oddity.

For most men, at some point in their lives, pornography has held a strong appeal and, before any examination of its effects, this fact has to be addressed. Like many men, I first saw pornography during puberty. At boarding school, dog-eared copies of Mayfair and Knave were stowed behind toilet cisterns; this borrow-and-return library system was considered absolutely normal, seldom commented upon and either never discovered by the masters or tacitly permitted. Long before my first sexual relationship, porn was my sex education.

No doubt (though we'd never have admitted it then) my friends and I were driven to use porn through loneliness: being away from home, we longed for love, closeness, unquestioning acceptance. The women over whom we masturbated - the surrogate mothers, if you like - seemed to be offering this but, of course, they were never going to provide it. The untruths it taught me on top of this disappointment - that women are always available, that sex is about what a man can do to a woman - I am only now, more than two decades on, finally succeeding in unlearning.

From men everywhere come similar stories. Nick Samuels, 46, an electrical contractor from Epping - now, with a wife and four children, the very image of respectable fatherhood - says he first discovered the power of pornographic images at the age of 16, when he found a copy of Mayfair in his father's garage. "I can even remember the picture. There was a woman walking topless past a building site and the builders were ogling her from the scaffolding. It was pretty soft stuff, but it heightened my senses and kicked off my interest in pornography. Before long, I was reading Whitehouse and then, through a friend at my squash club, I was introduced to hardcore videos."

Si Jones, a 39-year-old north London vicar who regularly counsels men trying to "come off" pornography, admits that, for him, too, it was his introduction to sex. "As a teenager, I watched porn films with my friends at the weekend. It was just what you did. It was cool, naughty and everyone was doing it." Set against today's habit of solitary internet masturbation, Jones's collegiate introduction to porn seems peculiarly sociable. Today, boys no longer clandestinely circulate magazines after school; nor do they need to rummage through their fathers' cupboards in search of titillating material. Access to internet pornography has never been easier, its users never younger, and the heaviest demand, according to research published in the New York Times, is for " 'deviant' material including paedophilia, bondage, sadomasochism and sex acts with various animals".

At its most basic level, pornography answers natural human curiosity. Adolescent boys want to know what sex is about, and porn certainly demonstrates the mechanics. David Morgan, consultant clinical psychologist and psychoanalyst at the Portman Clinic in London, which specialises in problems relating to sexuality and violence, describes this phase as "transitional, like a rehearsal for the real thing. The problem with pornography begins when, instead of being a temporary stop on the way to full sexual relations, it becomes a full-time place of residence." Morgan's experience of counselling men addicted to porn has convinced him that "the more time you spend in this fantasy world, the more difficult it becomes to make the transition to reality. Just like drugs, pornography provides a quick fix, a masturbatory universe people can get stuck in. This can result in their not being able to involve anyone else."

For most men, the way pornography objectifies sex strikes a visceral emotional chord. Psychotherapists Michael Thompson and Dan Kindlon, in their book Raising Cain: Protecting The Emotional Life Of Boys, suggest that objectification, for boys, starts early. "By adolescence, a boy wakes up most mornings with an erection. This can happen whether he is in a good or bad mood, whether it is a school day or a weekend ... Boys enjoy their own physical gadgetry. But the feeling isn't always, 'Look what I can do!' The feeling is often, 'Look what it can do!' - again, a reflection of the way a boy views his instrument of sexuality as just that: an object. What people might not realise when they justly criticise men for objectifying sex - viewing sex as something you do, rather than part of a relationship - is that the first experience of objectification of sexuality in a boy's life comes from his experience of his own body, having this penis that makes its own demands."

But the roots go back further still. Research has shown that boy babies are treated more harshly than their female counterparts and, as they grow up, boys are taught that success is achieved through competition. In order to deal with this harsh masculine world, boys can learn not to trust their own feelings and not to express their emotions. They become suspicious of other men, with whom they're in competition, after all, and as a result they often feel lonely and isolated.

Yet men, as much as women, hunger for intimacy. For many males, locked into a life in which self-esteem has grown intrinsically entwined with performance, sex assumes an almost unsustainable freight of demands and needs. Not only does the act itself become almost the only means through which many men can feel intimate and close, but it is also the way in which they find validation. And sex itself, of course, cannot possibly satisfy such demands.

It is into this troubled scenario that porn finds such easy access. For in pornography, unlike in real life, there is no criticism, real or imagined, of male performance. Women are always, in the words of the average internet site, "hot and ready", eager to please. In real life, by contrast, men find women are anything but: they have higher job status, they demand that they be sexually satisfied, and they are increasingly opting to combine career and motherhood.

Men, say psychologists, also feel threatened by the "emotional power" they perceive women wielding over them. Unable to feel alive except when in relationships with women, they are at the same time painfully aware that their only salvation from isolation comes in being sexually acceptable to women. This sense of neediness can provoke intense anger that, all too often, finds expression in porn. Unlike real life, the pornographic world is a place in which men find their authority unchallenged and in which women are their willing, even grateful servants. "The illusion is created," as one male writer on pornography puts it, "that women are really in their rightful place and that there is, after all, no real and serious challenge to male authority." Seen in this light, the patently ridiculous pornography scenario of the pretty female flat-hunter (or hitch-hiker, driver with broken-down car, or any number of similar such vulnerable roles) who is happy to let herself be gang-banged by a group of overweight, hairy-shouldered couch potatoes makes perfect psychological sense.

The porn industry, of course, dismisses such talk, yet occasionally comes a glimmer of authenticity. Bill Margold, one of the industry's longest-serving film performers, was interviewed in 1991 by psychoanalyst Robert Stoller for his book Porn: Myths For The Twentieth Century. Margold made no attempt to gloss over the realities. "My whole reason for being in this industry is to satisfy the desire of the men in the world who basically don't care much for women and want to see the men in my industry getting even with the women they couldn't have when they were growing up. So we come on a woman's face or brutalise her sexually: we're getting even for lost dreams."

As well as "eroticising male supremacy", in the words of anti-porn campaigner John Stoltenberg, pornography also attempts to assuage other male fears, in particular that of erection failure. According to psychoanalytical thinking, pornography answers men's fetishistic need for visual proof of phallic potency. Lynne Segal, professor of psychology and gender studies at Birkbeck College, University of London, writes: "Men's specific fears of impotence, feeding off infantile castration anxiety, generate hostility towards women. Through pornography, real women can be avoided, male anxiety soothed and delusions of phallic prowess indulged, by intimations of the rock-hard, larger-than-life male organ."

Pornography, in other words, is a lie. It peddles falsehoods about men, women and human relationships. In the name of titillation, it seduces vulnerable, lonely men - and a small number of women - with the promise of intimacy, and delivers only a transitory masturbatory fix. Increasingly, though, men are starting to be open about the effect pornography has had upon them. David McLeod, a marketing executive, explains the cycle: "I'm drawn to porn when I'm lonely, particularly when I'm single and sexually frustrated. But I can easily get disgusted with myself. After watching a video two or three times, I'll throw it away and vow never to watch another again. But my resolve never lasts very long." He has, he says, "seen pretty much everything. I've even seen pictures of men being buggered by a pig. But once you start going down that slope, you get very quickly jaded."

Like many men, McLeod is torn. Quick to claim that porn has "no harmful effects", he is also happy to acknowledge the contradictory fact that it is "deadening". Andy Philips, a Leeds art dealer and, at 38, a father for the first time, says there have been times when he has been "a very heavy user". His initial reaction, like that of many of the men to whom I spoke, is studiedly jokey: "I love porn." Yet, as he grows more contemplative, he admits: "I've always used it secretly, never as part of a relationship. It's always been like the other woman on the side. It's something to do with being naughty, I guess."

Again and again, despite now being married, he is drawn back. "You can easily get too much of it. It's deadening, nullifying, gratuitous, unsatisfying. At one point I was single for three years and I used a lot of porn then. After a while, it made me feel worse. I'd feel disgusted with myself and have a huge purge."

Extended exposure to pornography can have a whole raft of effects. By the time Nick Samuels had reached his mid-20s, it was altering his view of what he wanted from a sexual relationship. "I used to watch porn with one of my girlfriends, and I started to want to try things I'd seen in the films: anal sex, or threesomes." Sometimes, he says, this was OK - "She was an easy-going person." At other times, "it shocked her". Married for 15 years, he admits he has carried the same sexual expectations into the marital bedroom. "There's been real friction over this: my wife simply isn't that kind of person. And it's only now, after all these years, that I'm beginning to move on from it. Porn is like alcoholism: it clings to you like a leech."

Psychoanalyst Estela Welldon, author of the classic text Mother, Madonna, Whore, has treated couples for whom such scenarios spiralled out of control. "A lot of men involve their partners in the use of porn. Typically, they will say, 'Don't you want a better sex life?' I have seen cases in which first the woman has been subjected to porn and then they have used their own children for pornographic purposes." When couples use porn together - a growing trend, if anecdotal evidence is anything to go by - there is, says Welldon, "an illusory sense that they are getting closer together. Then they film themselves having sex and feel outside themselves. This dehumanising aspect is an important part of pornography. It dehumanises the other person, the relationship, and any intimacy."

Even when in a loving sexual relationship, men who have used porn say that, all too often, they see their partner through a kind of "pornographic filter". This effect is summed up eloquently by US sociologist Harry Brod, in Segal's essay Sweet Sorrows, Painful Pleasures: "There have been too many times when I have guiltily resorted to impersonal fantasy because the genuine love I felt for a woman wasn't enough to convert feelings into performance. And in those sorry, secret moments, I have resented deeply my lifelong indoctrination into the aesthetic of the centrefold."

Running like a watermark through all pornography use, according to Morgan at the Portman Clinic, is the desire for control. This need, he says, has its roots in early childhood. "A typical example might be a boy with fairly absent parents, either in emotional terms or in actual fact." The boy, wishing his parents were more present - more within his control, as it were - can grow up wishing "to find something over which he can have control. Pornography fills that space."

But the user of pornography is also psychologically on the run, Welldon adds. "People who use pornography feel dead inside, and they are trying to avoid being aware of that pain. There is a sense of liberation, which is temporary: that's why pornography is so repetitive - you have to go back again and again."

Lost in a world of pornographic fantasy, men can become less inclined, as well as increasingly less able, to form lasting relationships. In part, this is due to the underlying message of pornography. Ray Wyre, a specialist in sexual crime, says pornography "encourages transience, experimentation and moving between partners". Morgan goes further: "Pornography does damage," he says, "because it encourages people to make their home in shallow relationships."

Jan Woolf believes it might also prevent a relationship getting started. A former special needs teacher, she lasted only six months in the job of BBFC censor in 2001. During this time, she watched hundreds of hours of hardcore videos. At the time, she was single. "If I'd been in the early stages of a relationship, it would have been very difficult, because I'd have been watching what I might have been expected to be doing, except it would never have been like that." She left the job because the porn was starting to make her feel "depressed - I wanted my lively mind back".

The more powerful the sense of pre-existing internal distress, the more compelling becomes the pull towards pornography. For John-Paul Day, a 50-year-old Edinburgh architect in his first "non-addictive" sexual relationship, the experience of being a small boy with a dying mother drove him to seek solace in masturbation. He says he has been "addicted" to pornography his entire adult life. "The thing about it is that, unlike real life, it is incredibly safe," Day says. "I'm frightened of real sex, which is unscripted and unpredictable. And so I engage in pornography, which is totally under my control. But, of course, it also brings intense disappointment, precisely because it is not what I'm really searching for. It's rather like a hungry person standing outside the window of a restaurant, thinking that they're going to get fed."

Day, who has attended meetings of Sex Addicts Anonymous for 12 years, says, "Pornography is central to my own sex addiction in as much as sex addiction has to do with the use of fantasy as a way of escaping from reality. Even in my fantasies about 'real' people, I am really transforming them into pieces of walking pornography. It is not the reality of who they are that I focus on, but the fantasy I project on to them."

Like drugs and drink, pornography - as Day has realised - is an addictive substance. Porn actor Kelly Cooke, one of the stars of Pornography: The Musical, says this applies on either side of the camera: "It got to the point where I considered having sex the way most people consider getting a hamburger. But when you try to give it up - that's when you realise how addictive it is, both for consumers and performers. It's a class A drug, and it's hell coming off it."

The cycle of addiction leads one way: towards ever harder material. Morgan believes "all pornography ends up with S&M". The now-infamous Carnegie Mellon study of porn on the internet found that images of hardcore sex were in far less demand than more extreme material. Images of women engaging in acts of bestiality were hugely popular, the most frequently downloaded being of a brunette with - in the pornographer's trusty lexicon - "a huge horse cock in her tight pussy".

The mechanics of the pornographic search - craving, discovery of the "right" image, masturbation, relief - makes it, says Morgan, work like "a sort of drug, an antidepressant". The myth about porn, as a witness told the 1983 Minneapolis city council public hearings on it, is that "it frees the libido and gives men an outlet for sexual expression. This is truly a myth. I have found pornography not only does not liberate men, but on the contrary is a source of bondage. Men masturbate to pornography only to become addicted to the fantasy. There is no liberation for men in pornography. [It] becomes a source of addiction, much like alcohol. There is no temporary relief. It is mood-altering. And reinforcing, ie, 'you want more' because 'you got relief'. It is this reinforcing characteristic that leads men to want the experience they have in pornographic fantasy to happen in real life."

In its most severe form, this can lead to sexual crime, though the links between the two remain controversial and much argued-over. Wyre, from his work with sex offenders, says, "It is impossible not to believe pornography plays a part in sexual violence. As we constantly confront sex offenders about their behaviour, they display a wide range of distorted views that they then use to excuse their behaviour, justify their actions, blame the victim and minimise the effect of their offending. They seek to make their own behaviour seem normal, and interpret the behaviour of the victim as consent, rather than a survival strategy. Pornography legitimises these views."

One of the most extreme examples of this is Ted Bundy, the US serial sexual murderer executed for his crimes in January 1989. The night before his death, he explained his addiction to pornography in a radio interview: "It happened in stages, gradually ... My experience with ... pornography that deals on a violent level with sexuality is that, once you become addicted to it, and I look at this as a kind of addiction like other kinds of addiction, I would keep looking for more potent, more explicit, more graphic kinds of material. Like an addiction, you keep craving something which is harder, harder, something which gives you a greater sense of excitement, until you reach the point where the pornography only goes so far ... It reaches that jumping-off point where you begin to wonder if, maybe, actually doing it will give you that which is beyond just reading about it or looking at it."

Bundy, as damaged as he was, stopped short of blaming pornography for his actions, though it was, he believed, an intrinsic part of the picture. "I tell you that I am not blaming pornography ... I take full responsibility for whatever I've done and all the things I've done ... I don't want to infer that I was some helpless kind of victim. And yet we're talking about an influence that is the influence of violent types of media and violent pornography, which was an indispensable link in the chain ... of events that led to behaviours, to the assaults, to the murders." In the understated words of Wyre, "The very least pornography does is make sexism sexy."

The average man, of course, whatever his consumption of pornography, is no Bundy. Yet for those who have become addicted, the road to a pornography-free life can be long and arduous. Si Jones advises accountability: "Make your computer accountable, let other people check what you've been looking at."

And the alternative to pornography, says Morgan, is not always easy. "Relationships are difficult. Intimacy, having a good relationship, loving your children, involves work. Pornography is fantasy in the place of reality. But it is just that: fantasy. Pornography is not real, and the only thing human beings get nourishment from is reality: real relationships. And, anyway, what do you want to say when you get to the end of your life? That you wish you'd spent more time wanking on the internet? I hardly think so."