Exactly what has love got to do with it, Mum?

When he was about seven, Jim discovered hardcore European pornography in his father's wardrobe, and he can remember some of those first images he saw. "I found them quite disturbing. I couldn't talk to anyone about it, of course, because the whole point is that it's hidden. You know that you're not supposed to know about it."

From then on he would get up before his parents woke, before six in the morning, to look through his father's briefcase and find the porn magazines. "Then my dad got a Super 8 projector, when I was about 11 or 12, and he would hire porn films. He would lock himself in the dining room to watch them. But the real change came when he got a video, and I persevered till I found the films. I was about 14 and I would find them and watch them when I was alone in the house. Constantly."

Porn has been so normalised that anyone objecting to it now is just going to be laughed at. I think we need to hear again about how pornography threatens intimacy

At this age, Jim did not have any relationships to set against this obsession. He was going to a boys' school and never met girls socially. "I was obsessed with pornography, I wanted to be pornography, I wanted to live pornography," he said. "It wasn't good for me, I can see that now.

"I knew that even then, I think, but it was an addiction from the start. It had such a powerful hold on me. It had a huge effect on my behaviour with women.

"I was unable to think of women except as potential pornography. I looked at them in a purely sexual way.

"I remember one day I was walking to school, I was about 15, and I got talking to a girl who must have been about 18. I immediately said I wanted to grope her breasts. I had no idea how to interact with women as people."

Even though Jim began to have girlfriends from the age of 19, he never managed to shrug off the power of the fantasy world. "The power of pornography has continued throughout my adult life. Nothing has really measured up to the world of porn, for me. I've seen thousands of strangers having sex. So when I have sex, I am watching myself having sex."

In his thirties, he started a relationship with Ali, a direct-talking, well-read woman. He told her about his interest in porn and they used to watch videos together. At first she could see the "high" but when she became uncomfortable, he agreed to try to abstain. Once the internet was part of their lives, he could no longer control himself and began to use pornography again. The relationship broke down after seven years. "Pornography has made him only able to see sex one way," Ali said. "He has always seen sex as something that has to be performed, not felt."

She would like to see a public debate about the effects of pornography. "Porn has been so normalised that anyone objecting to it now is just going to be laughed at. I think we need to hear again about how pornography threatens intimacy."

For Jim, pornography "has destroyed my ability to have intimate relationships". One might think that someone who has seen as much as he has would not be unsettled by anything, but he is shocked by the way that the growing acceptability of pornography is putting into the mainstream a dehumanising view of women.

He finds the internet - with its images of rape, incest and abuse - "quite disturbing". He said: "The stuff I saw as a kid was what we called hardcore, but the idea in the text alongside was that it was based on mutual consent - mutual pleasure - but what I see now is more male domination."

Jim believes that very young men are beginning to see as normal images that would once have been seen as far beyond the pale. "It's like bravado, they want to look at worse and worse stuff. When I was a kid what you saw was limited by what you could physically buy on paper. Now it all flashes around so quickly and the taboos have just fallen."

Jim feels that, even for young men who don't seek it out, the exposure to these images simply changes their attitudes to sex. "I think that kind of violence associated with sex lodges in your mind and you never forget it, however much you want to. It's always there."

Not only is the tone of pornography so often reliant on real or imaginary abuse of women, it is consumed in increasing numbers by young people who have little real experience to set against it.

Ali worries that what happened with Jim could be repeated with her own son. "I was first aware that he was looking at pornography when he was 14. But how can boys not see it? Unless they make a concerted decision not to look at it, to delete it from their mobiles when it's sent to them, or from their emails. You'd be making a singular, probably a unique decision.

"Once someone like Jim was unusual, now every boy has seen all of that. I know what it does to young minds, and now it is more and more prevalent. God knows how we can begin to challenge this. Once upon a time, kids could experiment, you know, privately, but now all the innocence is lost."

For a long time I was sceptical about the claim that the internet had really changed people's access and attitudes to pornography. Those who want it have surely always been able to find it, whether they were living in 5th-century Athens or the 1950s. But the evidence has convinced me that the internet has driven a real change for many people, especially younger people.

Once upon a time, someone who was truly fascinated by pornography might have found, with some difficulty, 10, or 20, or 100 images to satisfy themselves. Now anyone can click on a single website and find 20, 100, 1,000 choices of videos and images, with the most specialist and violent next to the most gentle and consensual.

Before they have touched another person sexually or entered into any kind of sexual relationship, many children have seen hundreds of adult strangers having sex

Statistics tell a story that is hard to ignore. A survey carried out in 2006 found that one in four men aged 25-49 had viewed hardcore online pornography in the previous month and that nearly 40% of men had viewed pornographic websites in the previous year.

It is the prevalence of pornography consumption among children that is most striking. In a study in 2000, 25% of children aged 10-17 had seen unwanted online pornography in the form of pop-ups or spam. By 2005 the figure was 34% - and 42% of children aged 10-17 had seen pornography, whether wanted or unwanted. In another study in Canada, 90% of boys aged 13 and 14 and 70% of girls the same age had viewed pornography. Most of this porn use had been over the internet. More than one-third of the boys reported viewing pornographic DVDs or videos "too many times to count".

While once someone could live their whole lives without ever seeing anyone but themselves and their own partners having sex, now the voyeur's view of sex has been normalised, even for children.

For an increasing number of young people, pornography is no longer something that goes alongside sex but something that precedes sex. Before they have touched another person sexually or entered into any kind of sexual relationship, many children have seen hundreds of adult strangers having sex.

When I spoke to one teenager who is studying for his A-levels and quoted statistics to him that said that the majority of young teenagers have looked at pornography, he laughed.

"More like 100%," he said. "It's when you're 13 and 14 that everyone starts looking and talking about it at school - before you're having sex, you're watching it.

"I think that those lads' mags are only read by certain kinds of boys. My friends wouldn't read them, to be honest, just like they wouldn't buy The Sun. But pornography - it crosses every social class, every cultural background.

"Everyone watches porn. And I think that's entirely down to the internet; not just your home computer, but everything that can connect - your phone, your BlackBerry, whatever you've got - everyone's watching porn.

"Adults have got to know what teenagers are doing, and if you're caught, you get told off. But I never had a serious discussion with a teacher or anyone about it."

I heard from teenagers that they want more chance to discuss seriously what they are seeing, since they seem to find that this world of pornography is absolutely open to them and yet is rarely referred to openly.

Now that the classic feminist critique of pornography - that it necessarily involves or encourages abuse of women - has disappeared from view, there are few places that young people are likely to hear much criticism or even discussion about its effects.

Many women who would call themselves feminists have come to accept that they are growing up in a world where pornography is ubiquitous and will be part of almost everyone's sexual experiences. I can see why some are arguing that the way forward really rests on creating more opportunities for women in pornography, yet I think it is worth looking at why some of us still feel such unease with the situation as it is now.

I do not believe that all pornography inevitably degrades women, and I do see that the classic feminist critique of pornography is too simplistic to embrace the great range of explicit sexual materials and people's reactions to them. Yet let's be honest. The overuse of pornography does threaten many erotic relationships, and this is a growing problem. What's more, too much pornography does still rely on or promote the exploitation or abuse of women. Even if you can find porn for women and couples on the internet, nevertheless a vein of real contempt for women characterises so much pornography.

The massive colonisation of teenagers' erotic life by commercial pornographic materials is something that it is hard to feel sanguine about. By expanding so much in a world that is still so unequal, pornography has often reinforced and reflected the inequalities around us.

This means that men are still encouraged, through most pornographic materials, to see women as objects, and women are still encouraged much of the time to concentrate on their sexual allure rather than their imagination or pleasure. No wonder we have seen the rise of the idea that erotic experience will necessarily involve, for women, a performance in which they will be judged visually.

When I interviewed young women about their attitudes to sexuality, I was struck by one apparently trivial fact: that all of them agreed that they would never want to have sex if they hadn't depilated their pubic hair.

"I would never want a man to see me if I hadn't been waxed recently," said one young woman from Cambridge University, and her friends nodded in agreement. "I don't need to have all the hair removed, but it has to be neat," said another.

"That is definitely tied into porn," said another. "We know what men will have seen and what they will expect."

Where the rise of expectations from pornography result just in depilation, that is one thing, but the rise of interest in surgery to change the appearance of the labia is another, far more worrying development. The number of operations carried out in the UK to cut women's labia to a preconceived norm is currently rising steeply.

It is true that we cannot turn back the clock and wipe pornography out of our individual experience or the memories of our society. Yet there are still ways to move forward and to create places where the influence of pornography will be resisted

This development has been covered extensively in magazines and television programmes, often in a way calculated to increase anxiety among female viewers.

In an episode of Embarrassing Teenage Bodies, screened on Channel 4 in 2008, a young woman consulted a doctor about the fact that her labia minora extended slightly beyond her labia majora and that this caused her embarrassment. Instead of reassuring her that this was entirely normal, the doctor recommended, and carried out, surgery on her labia.

The comments left on the programme's website showed how this decision to carry out plastic surgery to fit a young woman's body to a so-called norm made other young women feel intensely anxious.

"I'm 15 and I thought I was fine, but since I've watched the programme I've become worried, as mine seem larger than the girl who had hers made surgically smaller! It doesn't make any difference to my life, but I worry now that when I'm older and start having sex I might have problems!" one girl said.

This idea that there is one correct way for female genitals to look is undoubtedly tied into the rise of pornography. One website for a doctor who specialises in this form of plastic surgery makes this explicit: "Laser reduction labioplasty can sculpture the elongated or unequal labial minora (small inner lips) according to one's specification ... Many women bring us Playboy and say that they want to look like this. With laser reduction labioplasty, we work with women to try to accomplish their desires."

If the rise of pornography was really tied up with women's liberation and empowerment, it would not be increasing women's anxiety about fitting into a narrow physical ideal.

The tide of pornography is so huge, and so easily accessible, that it often seems impossible to think about turning it back. Yet I don't think we have to slip into despair. There is this idea that "innocence", once lost, is lost for ever, that, as Jim put it, once pornography is viewed, "You never forget it, however much you want to."

It is true that we cannot turn back the clock and wipe pornography out of our individual experience or the memories of our society. Yet there are still ways to move forward and to create places where the influence of pornography will be resisted. This will entail giving more support to people who are struggling with its dehumanising effects on their own relationships.

The starting point is public debate. A woman I'll call Lara, who has been trying for several years to persuade her husband to give up pornography, wrote to me: "From some discussions I've had online I can see that many wives are struggling with their husband's porn use. If the mainstream media began talking about porn addiction in the same way as they talk freely about drug abuse, gambling or alcoholism, then maybe my husband would see that he's not the only man in the world who has this problem and would see that he should deal with it."