In just one generation, the land of the stiff upper lip has become the land of the permanent stiffy. Only a few decades ago, Kingsley Amis and Philip Larkin would lament the unavailability of porn in their letters to each other, and share the odd dull nude photograph. Today, a tide of bodily fluids leaks into every e-mail inbox; nobody is more than a click away from "Hard-Core Fuck Action!!!" and "Barely Legal Bitches Free Click Here". The most-watched film in Britain last year was not the latest Lord of the Rings or Harry Potter flick. No: the winner by a spurt was the stolen home movie in which the former Blue Peter presenter John Leslie has sex with Abi Titmuss - downloaded by six million of us. The language of net porn - "twinks", "bears", "bitches" - can be heard in every pub, playground and post-office queue.
Even the most frigid Brit can spot the sexual differences. The mainstreaming of hard-core porn is now splashing on to Britain's high streets and into town centres. Vibrators are sold in Boots, the spiritual home of middling Middle England. The obscenity laws lie lifeless on the statute book. British sexual exhibitionism pokes forward from Ibiza to the nation's 200 dogging sites. (If you don't know what dogging means, it is, like every other sexual act on earth, just a Google away.)
Yet there is still one lingering leftover from our Puritan past: the sudden ubiquity of porn has barely been discussed in public. Only 20 years ago, there was a public ruckus - sparked by Clare Short - about page-three girls. Today, anyone can see those same girls being (to use the language of the web) spit-roasted and gang-banged. Yet if your only sources of information were the press, TV shows and Hansard, you would have no idea.
The release of a Hollywood biopic of Alfred Kinsey, the sex researcher whose writings shocked 1940s and 1950s America, helps us to realise how far we have come (and, indeed, cum). Kinsey's agenda was simple and brave: to eradicate sexual shame and melt sexual taboos. His own life was littered with the victims of Judaeo-Christian sexual repression. Kinsey's father was a bullying, violently repressed bigot, in part because he was brutally punished for masturbating when he was an adolescent. As a grown man and the first serious modern student of human sexuality, Kinsey spent his life documenting the guilt and samizdat sex of his time. He discovered a world of married couples who knew nothing but the missionary position; of grown women who did not know where their clitoris was, or that it even existed; of gay people who despised their own sexual urges. But today - thanks largely to the web - we are beginning to live in the world Kinsey sought. It is a place where no sexual act remains in the shadows and where - as one of his students puts it in the film - "fucking is nothing more than friction and harmless fun".
Yet the few people who have tried to discuss this new age of porn are stuck with a batch of sterile, rote-learned positions that have not evolved since the early 1970s. This triumvirate can be dubbed the Christian Puritans, the Feminist Puritans and the Libertines.
Leslee J Unruh is the poster girl for the Christian anti-porn mullahs. She is the sassy, aggressive leader of America's Abstinence Clearinghouse, an evangelical group that has been leading pickets of Kinsey across the US.
"Kinsey should be looked upon in the history books as Hitler, as Saddam Hussein," she explained last month, adding that internet porn is "turning America into Sodom and Gomorrah". This voice has become louder and clearer since George W Bush's "moral values" re-election, and it has its British representatives in Melanie Phillips and the Daily Mail.
By contrast, the Feminist Puritans - still epitomised by the dungaree-wearing radical Andrea Dworkin - recoil not at the "filth" of porn but at its misogyny. They describe it as "a way of sexually enslaving women", a system of "male truth" that is "forced on women and wrecks their lives".
And the Libertines? They are simply the mirror image of the anti-porn hysterics. From disciples of Kinsey such as Gore Vidal to "pro-porn feminists" such as Camille Paglia, they un-questioningly celebrate the rise of digital porn as a glorious Dionysian orgy after two millennia of unnatural Christian repression.
And that's it. Those three shagged-out arguments represent our shallow response to the new global peep-show. Yet looking around at people who are (like me) in their twenties, I see porn shaping us in a thousand ways, none of which can be answered by reading from these sagging scripts. There are advances that would make Kinsey hard with joy. Today's teens and twentysomethings are probably the most sexually literate in European history. Many use the web as a digital Kama Sutra and absorb more forms of rewarding sexual expression from it than our grandparents could ever have dreamed of.
But I also see victims. There are the men who describe themselves as "chronic porn addicts" and admit that the presence of ten million fantasy women forever splayed in the corner of their flat has warped their emotional lives. There are the teenage lads with wildly unrealistic expectations that women are constantly "up for it" and infinitely sexually pliable. And there are girls trying to meet those swollen expectations - girls who have internalised the norms of pornography and who try to convince themselves that they enjoy their boyfriends' endless requests for anal sex, sex toys and being "shared" with the mates.
None of these changes can be understood with reference to the spent debate about whether porn is a demon or saviour. It's a pointless argument. We cannot even eradicate child pornography, which is universally and rightly abhorred; how could we ever contemplate eliminating porn itself?
Instead, we need to discuss how we can equip ourselves for life in this new Kinseyan age. Kinsey, like the early 20th-century sex reformers such as George Bernard Shaw and Marie Stopes, believed that freedom would act as a balm, soothing extreme forms of sexual activity including prostitution, paedophilia and rape. They certainly did not believe it would lead to more extreme sexualities. Yet that seems to be the effect of growing sexual freedom. Among the straight male friends who spoke to me for this article, the recurring theme was that internet porn had led them to explore more unusual and (to them) disturbing forms of sexual expression. As one put it, "You just get bored with a parade of ordinary-looking vaginas. After a while, I found myself looking up things I would never have imagined I would want to see."
The evidence for this is not just anecdotal. The psychologist Jennings Bryant has studied the effects of sustained exposure to non-violent pornography - and found that men rapidly begin to seek increasingly extreme material as their exposure to porn increases. He found that "pornography can transform a male who was not previously interested in the more abusive types of pornography into one who is turned on by such material". Some of this went in very disturbing directions: the Canadian psychologists James Check and Ted Guloien have conducted an extensive study of the effects on ordinary men of sustained exposure to imagery depicting rape. They found that the men's internal inhibitions against committing rape were significantly undermined, and they were more likely after seeing the imagery to trivialise or condone rape. Both of these effects have been replicated in dozens of studies. So will one of the features of this new age - in addition to the welcome growth in sexual openness - be a terrible wave of increased sexual assaults?
A 17-year-old - a product of the age - suggested one way to halt this. "Young men need to be taught from adolescence to be porn-savvy. Everybody knows from the time they're a child to be sceptical of the claims of advertising, but young lads don't know to be sceptical about the claims involved in porn. My first experience of women in a sexual context was seeing them on websites as cum-hungry bitches. I guess I started looking at it when I was 11 or 12, and it led me to make some terrible mistakes, approaching girls and expecting them to be into anything and everything. The sex education we got was like something from another age. We were told in class what a vulva was when I was 14, but by that time I had been inspecting them in detail on my computer screen for years, and so had every other lad in the room. I knew what they looked like; what I didn't know was that there was such a huge emotional gap between porn and reality. That's what they need to teach."
Are we going to carry on pretending in schools and in public that the internet sex show doesn't exist? Do we want to prepare ourselves for this powerful and warping intoxicant, or do we want to remain unsheathed and unprepared? When it comes to hard-core porn, we are - to borrow Jean-Paul Sartre's ambiguous phrase - condemned to be free.