Explicit sex in films and books used to be shocking but there are signs of a new liberalism gaining ground. This autumn John Cameron Mitchell's film Shortbus - which shows unsimulated sexual relationships in all their fun and intimacy - opened in America without a peep from anyone. Are we growing up at last?

You could say it starts with a bang. John Cameron Mitchell's film Shortbus begins with a looping tracking shot over a cardboard cutout of Manhattan. Then his voyeur camera looks inside three apartment windows. In one, a young man is in a yoga position up on his shoulders, his legs arched above his head, slowly fellating himself to ejaculation. In another, a couple are having the noisiest every-which-way sex, to no obvious end. In the third, a student is being humiliated by a dominatrix, to such an explosive degree that he eventually adds his own DNA squiggle to a Jackson Pollock print on the far wall. Each of these scenes sets the tone for what follows: an extraordinarily graphic, extremely funny and touchingly human film about the tragi-comedy of sex.

The character of any age is tellingly revealed in the popular representation of intimacy. For all the sexualisation of our culture, we live in strangely repressed times: a late-night, infrared fumble on Big Brother is front-page news. While the online porn industry, with its humourless siliconed stereotypes, is worth a reported $10bn a year - more than the cumulative box-office receipts of Hollywood - real human sexual relationships, vulnerable and fun, are hardly anywhere to be seen.

One of the achievements of Shortbus, once you have got used to the unsimulated sex (an adjustment that, after the opening, happens surprisingly quickly) is to make you realise how odd it is that such a film has not been made before: why make romantic comedies for adults that stop at sex, when so much of the comedy and drama of romance occurs because of it?

Real sex on mainstream screens is no longer new or shocking in itself. Since the censors at the British Board of Film Classification relaxed guidelines in a surge of fin de siecle liberalism, there have been several films that have created (and courted) headlines by depicting honest couplings: Michael Winterbottom's 9 Songs, most of which fell flat, opened a debate on the subject; Catherine Breillat's misanthropy in Romance prefigured the manipulative sexual violence of Baise-Moi ('rape me'); Patrice Chereau's Intimacy broke new ground by having name actors - Kerry Fox and Mark Rylance - getting it together.

In all these cases, however, the sex was doomed, furtive or violent; usually a symbol of loneliness rather than connection, a guilty secret. The films reflected that tendency, taken to extremes in the novels of Michel Houellebecq, to suggest that all modern sexuality is desperate or self-loathing. Houllebecq set out his stall in his first novella, Whatever: 'In reality,' he wrote, as a kind of manifesto, 'the successive sexual experiences accumulated during adolescence undermine and rapidly destroy all possibility of projection of an emotional and romantic sort; progressively and, in fact, extremely quickly, one becomes as capable of love as an old slag. And so one leads, obviously, a slag's life; in ageing, one becomes less seductive and on that account bitter. One is jealous of the young, and so one hates them...' And so on.

In Porno, his sequel to Trainspotting, Irvine Welsh picked up this fashionably prevailing mood and suggested that video and the internet have made us all into addicted voyeurs. The growth in DIY porn is the new drugs, he argued: kids are no longer shooting up but shooting footage of themselves having sex and then showing these films in the back room of their local, or, more likely, posting them on websites. Porn is 'like eating McDonald's,' Welsh wrote. 'People know it won't do them any good but it becomes compulsive; amateur porn is the new karaoke...'

Shortbus takes on such reductive arguments and attempts to find a way through them. The exhibitionists in the opening sequence are quickly revealed as real people, trying to make sense of themselves through sex. Mitchell's approach to them feels like something new because it is prepared to risk proper intimacy and is happy to be less than solemn about the results. Sex, the way people do it, what it says about them, becomes a character trait to explore rather than a symptom of a society in decay. Perhaps because of this, Shortbus has a regular 18 certificate and nationwide distribution.

In America, where it came out in October, it was universally applauded - Time magazine called it 'unmissable', the New York Times suggested that 'as Utopian visions go, it doesn't get much better than Shortbus,' and went on to say that Mitchell's film was a welcome blast of optimism 'at odds with an American movie mainstream that has grown progressively more prudish about sex over the last three decades, while its representations of violence have grown more obscenely violent. Hollywood says let it bleed. Mr Mitchell would rather we get off on life...' Unusually the public too seemed to get the message that, though explicit, the film was clearly not porn (the purpose of porn is simply to arouse, Mitchell argues, the sex here is a way of exploring the interior life of the characters). Most remarkably of all, perhaps, the film did not excite any censorious comment from the religious right; the anti-gay lobby stayed quiet, even Rush Limbaugh kept shtum.

It would be tempting to think that this was because America was finally getting a bit more grown up about sex, or because the nation at war with itself was ready for some frank hedonism.

When I met Mitchell in London a couple of days after I had seen his film, though, he offered an alternative explanation. He was a little disappointed about the fact his film had not excited picket lines of shocked Middle Americans, or fulminating editorials, but he put it down not to a new open-mindedness but to Republicans being 'busy losing the midterm elections and with their own sexual scandals'. He felt he had done his level best to stir up a little patriotic debate - in one memorable scene, the hindmost participant in a gay threesome sings a necessarily muffled version of 'The Star-Spangled Banner' - but there was nothing.

Maybe he was too successful in his overall aim, which he saw as 'reminding people that sex can be sweet. In the US in particular, sex is just porn or kind of wrist-slashing. We crush it from above; it's this multibillion-dollar industry we can't talk about; then Janet Jackson's tit pops out and all hell breaks loose...'

Mitchell believes British audiences may be even more readily disposed toward the film than American ones; the response at the London premiere, a standing ovation, matched that his film received at Cannes. 'Each country has a slightly different way of approaching how to get it out,' he says. 'Over here, they seem to be targeting straight females, maybe hoping they will drag the boyfriend along. Which is one way of doing it I guess.'

Mitchell seems an unlikely sexual revolutionary. Aged 42 and determinedly boyish, he is the son of an American general, who was commander of US forces in Berlin in the years leading up to 1989 and the fall of the wall. In his previous film, the remarkable Hedwig and the Angry Inch, he directed himself as an East Berlin transvestite and 'internationally ignored song stylist', survivor of a botched sex-change operation (hence 'the inch'). He picks up some of those 'cabaret' themes again in Shortbus, which is the name of a liberated club in Dumbo (Down Underneath the Manhattan Bridge Overpass) in Brooklyn, where the three characters introduced in his opening scenes eventually find some satisfaction.

Watching the film, in this knowledge, you can't help but wonder what his parents made of it. 'They are sort of horrified,' he admits. 'They are very pro-Bush and very conservative. People in the States tend to circle the wagons, especially lately.'

Have they watched Shortbus?

'No, they said, "Well, you didn't go to see The Passion of the Christ..." I pointed out that they did not make The Passion of the Christ. And anyhow, why would I prefer to go and see some three-hour film in which a man is flayed alive than two people having it off? That kind of says all you need to know about this stupid Western culture we live in. My film finally reached Colorado Springs, where my parents live, which is kind of where all the evangelicals end up. My mother emailed me to say they were thinking of going but there was a snowstorm... I emailed them, "Do not go to the movie."'

Mitchell is something of an evangelist himself, though in his case for a cheerful sexual frankness. When I am talking to him, singer Rufus Wainwright's plea for an American Second Coming in 'Gay Messiah' finds its way into my head: 'He will then be reborn/From 1970s porn/ Wearing tube socks with style/and such an innocent smile.' Shortbus enjoys just such a revivalist energy, which is perhaps, in Bush's polarised America, gaining more and more currency. At one point in the film, Mitchell has Justin King, the master of revels at an anything-goes house party, announce brightly: 'It is just like the Sixties, only with less hope.'

There is, in this sense, a determination about Mitchell to take up the challenge of Bertolucci's Last Tango in Paris or Oshima's Ai no corrida and make the screen once again a place where the in timacies of adult couples can be dramatised. It seemed, back in the Seventies, that the new realism that had come into film depictions of violence would naturally extend to sexuality. But then studios lost their nerve, special effects took over, Aids cast a shadow and properly complex sex came close to disappearing from Hollywood films. It was at that point that a sleazier, joyless version went underground and online, and low-budget pornography became America's biggest growth industry.

In this sense, I suggest to Mitchell, Shortbus is a kind of Sixties throwback, or at least a post-Aids, post-9/11 New York film.

'That wasn't completely conscious,' he says. 'But yes, there are other films that have been using sex and they came out after Aids was off the front page, at least in the Western world, and they were still unrelentingly grim. It almost became a cliche that the sex ended disastrously. I wanted to say it didn't have to be that way... '

The film's name, which Mitchell used for some of his own regular house parties, refers to the traditional American yellow school bus. Most 'normal' kids rode in a long yellow bus. Children with special needs - the disabled, the emotionally disturbed, the abnormally gifted - rode in a shorter yellow bus, because there were not so many of them. 'A lot of people I hang out with feel to me like they are familiar with the short bus in one way or another,' he says.

One of the things that had long militated against a Shortbus-style exploration of sex in films has been the star system; named actors have, justifiably, resisted going the extra mile on film (though rumours persist about, say, Donald Sutherland and Julie Christie in Don't Look Now or Halle Berry and Billy Bob Thornton in Monster's Ball).

Mitchell got around body doubles and nudity writers by finding his own stars. He made Shortbus on a $2m budget (some of it stumped up by singer Moby). 'I think of this film as kind of a Hollywood film from an alternate universe,' he says. 'Formally, it is quite traditional. The actors are charismatic, zesty; we are not going for kitchen-sink ordinariness. To me, it is an audience-friendly entertainment.'

Whether Mitchell's experiment will catch on is more debatable. In stark contrast to the brutal wham-bam schedules of porn films he has devoted nearly four years to his film; less one-night stand than long-term commitment.

To find his cast, he invited would-be actors to supply a 10-minute video in which they described a sexual experience that was important to them. He received 500 and sat through them all.

After he had assembled his cast - further auditions including a 100-person game of spin the bottle - he used improvisation ideas inspired by Robert Altman and Mike Leigh to develop characters and a story. He had no real idea of where he wanted it to go, beyond an upbeat kind of tone - which borrows something from Almodovar and even Woody Allen - and a sense of mischief ('Bush was on my mind,' he says). Mostly, the actors developed aspects of their real anxieties.

For a year and a half, Mitchell then worked closely with his cast to break down inhibitions and encourage a kind of emotional honesty. In some scenes, he had the cameramen and himself strip to make the actors feel less vulnerable; in one of the more populous orgies, he joined the fray.

I wonder how the idea for the gymnastic opening act cropped up?

He smiles. 'Well, on our original website, I had an image of someone blowing themselves and I used it as an example of how sex could be a metaphor for something else. It plays to this idea that we want to be alone, self-sufficient, almost self-fertilising. Anyway, I mentioned this and one of the actors, Paul DeBoy said, "You know I used to be able to do that." I said, "Do you think you could still do it?" It worked because his character was someone who was truly exploring what it means to be alone, ultimately in a possibly tragic way. Anyhow, he went to work practising, did his yoga; by the end, he could do it a little too well. I wondered, "Could you make it look a little more tricky?"'

The development of that character - the ultimate Houllebecqian figure, who initially films his own acts of masturbation - into someone capable of attachment is something close to a Mitchell manifesto. For all its explicitness, the film makes a good case for old-fashioned romance.

Mitchell hopes that, in a way, it's an antidote to a culture in which sex is increasingly just another online transaction. 'It is certainly an argument for getting bodies in the same room,' he says. 'The internet and all this new technology, these opportunities for connection, are at the same time opportunities to be bombarded into numbness... the YouTube, MySpace phenomenon is an example of people reaching out, desperate for intimacy.'

The dangers of those connections, he suggests, are that they offer 'a palliative, a sugar high' and create an addiction to constant newness. 'Kids are afraid now of the lag in between checking their email. You lose sight of the fact that you have to get down to get up, that you have to have boredom in a relationship in order to get to the next step.'

What all that suggests, he believes, and what his film makes clear, is that 'it's a natural human requirement actually to look into someone's eyes at close quarters, open yourself up. You don't get that from looking at a computer screen. Our reactions to that, anomie, alienation, depression, are symptoms of the mind telling us we have to change.'

It is hard to imagine many directors exactly following Mitchell's lead in this little sexual revolution but, nevertheless, Shortbus seems as good a place to start it as any.

· Shortbus opens nationwide on Friday

Sex on screen: A brief history

1912 The British Board Of Film Classification (BBFC) is founded; one of its first rules is no nudity.

1930 Will H Hays establishes the Hays Code, setting out what was morally acceptable in American films. Until the late Sixties all films adhered to the code.

1930 Louise Brooks stars as Lulu in GW Pabst's Pandora's Box - the first explicit lesbian character on screen.

1933 Red-Headed Woman, in which Jean Harlow plays a particularly sexually active woman accused of murder, is banned in Britain and not granted a certificate until 1965.

1959 A scene containing cunnilingus is cut from Louis Malle's film Les amants by the BBFC; the poster for the film is banned by London Transport.

1960 The Compton Cinema Club in Soho opens playing pornographic films.

1961 Stanley Kubrick's Lolita is released after Lolita's age is increased from 12 to 14.

1970 Male genitals are seen on screen for the first time, as Alan Bates and Oliver Reed wrestle naked in Ken Russell's Women in Love.

1970 Andy Warhol's Flesh is the first film to show an erection on screen.

1982 The R18 rating is introduced in Britain, allowing non-violent pornography to be shown in a club setting.

1989 The film Scandal is retouched to remove a couple who look like they are having real sex at an orgy.

2000 Darren Aronofsky loses his appeal against an NC-17 rating for his film Requiem for a Dream so releases it unrated.

2006 Kirby Dick releases This Film is Not Yet Rated, a documentary about film censorship in America, which receives a standing ovation when premiered at the Sundance Film Festival.