Thanks to the internet, porn has become central to our lives, with serious consequences for our sexual and emotional health

Andrea Dworkin, the anti-porn activist, rose to fame in the 1980s arguing that if we did not limit pornography most men would objectify women more intensively and treat them less as people than as porn stars. The floodgates would open; rape and other sexual transgressions would follow.

Since then the advent of the internet and, more importantly, broadband in most Western homes has meant that pornography has left the space that it once occupied of being a marginal, adult, private pursuit and has saturated a mainstream public arena. The whole world has become pornographised. A decade ago the "outing" of men's use of porn was often a scandal. But in the fallout from Jacqui Smith's expense claims, and the exposure of her husband, Richard Timney, for watching pay-per-view pornographic films, there has been no particular outrage about his use of porn. The scandal has been more that public funds settled the bill.

The unspoken assumption now is that everyone - at least, most men - use porn. A survey of British teenagers for Channel 4 revealed this week that 28 per cent learnt about sex from porn. Young men and women are indeed being taught what sex is, how it looks, what its etiquette and expectations are, by pornographic training. This is a giant shift for our species' imprinting about sex. For most of our history erotic images have been reflections of, or celebrations of, or substitutes for, real naked women. But today, for the first time in human history, the power and allure of the images have supplanted that of real naked women.

When I came of age in the Seventies it was still pretty cool to be able to offer a young man the actual presence of a naked, willing young woman. There were more young men who wanted to be with naked women than there were naked women on the market. If there was nothing actively alarming about you, you could get a pretty enthusiastic response merely by showing up. Your boyfriend may have seen Playboy, but, hey, you could move, you were warm, you were real. Thirty years ago simple lovemaking was considered erotic in the pornography that entered mainstream consciousness: clumsy, earnest, missionary-position intercourse was still considered to be a huge turn-on.

Well, I am 42, and mine is the last female generation to experience that sense of sexual confidence and security in what we had to offer. Now, simply being naked is not enough; you have to be buff, tanned - with no tan lines - have the surgically hoisted breasts and the Brazilian bikini wax, just like in the XXX-rated movies. The norms for Western beauty seem to be informed, whether it's intentional or not, by pornography. In my gym I notice that 40-year-old women have adult pubic hair but the twentysomethings have all been trimmed and styled - hardly a look derived from, say, the fashion world. In the 1970s porn was a bad substitute for real naked women. Today it is perfect porn that is "real" sex to young people - and real naked women are just bad porn.

Porn is no longer beyond the pale; the "cool girls" go with guys to the strip clubs and even ask for lap dances; college girls are expected to tease guys at keg parties with lesbian kisses. Does all this sexual imagery in the air mean that sex has been liberated?

Actually, Dworkin got it backwards. All this surfeit of cheap erotica is diluting sexual energy, especially for the young. The relationship between the multibillion-dollar porn industry, compulsiveness and sexual appetite has become like the relationship between agribusiness, processed foods, super-size portions and obesity. If your appetite is stimulated and fed by poor-quality material it takes more junk to fill you up. Research is showing that porn is indeed addictive, especially to men, and that it damages their libido in the longterm. Experts on sexual dysfunction are seeing an epidemic today of healthy young men who cannot perform easily with their partners because they have been overexposed to pornography. With increased exposure to porn many men need higher and higher levels of stimulation, or more and more extreme situations, in order to become aroused. So it seems that people are not closer, erotically, because of porn but less sexually connected.

Porn deadens desire. The response I got when I said as much in New York magazine recently showed that this really struck a chord with men. A whole generation may be less able to connect erotically to real women. I have met men who care about their sexuality who have moved away from porn not for moral reasons but, rather, for physical and emotional-health ones; they want to protect their desire.

The data of this epidemic should not be surprising. After all, pornography works in the most basic of ways on the brain. It is Pavlovian. An orgasm is one of the biggest reinforcers imaginable. If you associate orgasm with your wife, a kiss, a scent, a body, that is what, over time, will turn you on; if you open your focus to an endless stream of ever-more-transgressive images of cybersex, that is what it will take to arouse you. The ubiquity of sexual images does not free the power of Eros but dilutes it.

Feminists have often misunderstood sexual prohibition. I am not advocating a return to the days of hiding female sexuality, but I am noting that the power and charge of sex are maintained when there is some sacredness to it.

In some cultures it is not prudery that leads them to discourage men from looking at pornography. It is, rather, because these cultures understand male sexuality and what it takes to keep men and women physically interested in one another over time to help men, in particular, to, as the Old Testament puts it, "rejoice with the wife of thy youth; let her breasts satisfy thee at all times". These cultures urge men not to look at porn because they know that maintaining a powerful erotic bond between parents is a key element of a strong family.

I will never forget a visit I made to Ilana, an old friend who had become an Orthodox Jew in Jerusalem. She had abandoned her jeans and T-shirts for long skirts and a headscarf. "Can't I even see your hair?" I asked, trying to find my old friend in there. "Only my husband," she said with a calm sexual confidence, "ever gets to see my hair." When she showed me the bedroom, draped in Middle-Eastern embroideries that she shares only with her husband - no kids allowed - the sexual intensity in the air was archaic, overwhelming. It was private.

Compare that with a conversation I had with a student after I had talked about the effect of porn on relationships. "I prefer to have sex right away just to get it over with, to get rid of the tension."

"Isn't the tension kind of fun?" I asked. "Doesn't that also get rid of the mystery?"

"Mystery?" He looked at me blankly. And then, without hesitating, he replied: "I don't know what you're talking about. Sex has no mystery."