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It may well become one of the first dinner-party talking points of 2008: what did you think of the sex scenes in Ang Lee’s Lust, Caution? There are scenes so explicit it may well change the way other directors feel emboldened to approach sex in the future, and not just because we see Tony Leung throw female co-star Tang Wei across a bed and penetrate her from behind. Perhaps I imagined it – such is the power of Lee’s direction– but I swear you see Leung’s erect penis. In fact, I am told, you just see his testicles. Well, still . . .

Are these scenes necessary, or acceptable? Lee isn’t an exploitative director but the sex goes on for an epic amount of time in an epic amount of detail. But I would argue that these scenes are justified: taken in the context of the film, they directly reference Lee’s bigger concern, the tortured relations between China and Japan as reflected in the struggle and dangerous attraction between Leung and Wei. These are two people who are not what the other thinks they are, but have an honesty about sex that is far more revealing than either expected. The sex is so explosive because it is set in a country where intimacy of any kind is proscribed. For me, sex and explicit sex can be absolutely justified as a way of explaining character, motivation and plot.

The debate around sex and cinema, gratuitousness versus art, has raged since Marlon Brando flung Maria Schneider face down and reached for the butter in Last Tango in Paris, breaking not just the taboo of showing sex on the big screen but a sexual taboo itself – anal sex. Before then, Hollywood approached sex only gingerly while the censor prowled during the Thirties, Forties and Fifties; an era when sex was implied, rather than applied, in cinema.

Lauren Bacall could just about get away with telling Bogie to “just put your lips together and blow” in To Have and Have Not (1944); while The Big Sleep (1946) featured a number of scenes featuring suggestively smoking prostitutes. And God Created Woman (1956), featuring Brigitte Bardot, was considered outrageous at the time, and featured Bardot in lusty form.

Sex emerged as a joke in theCarry Onfilms of the Sixties and Seventies and in films such as Confessions of a Window Cleaner. Last Tango demanded that it be taken seriously; that the sado-masochism that Brando’s character enjoys explains him.

Another film of the same era, Nick Roeg’s Don’t Look Now (1973), features brilliantly cut scenes of Julie Christie and Donald Sutherland making love; and that again adds a layer to their Venice-set nightmare.

I know many viewers will find Lust, Caution unwatchable because it is so explicit. Doubtless many more will be offended. The Chinese censors trimmed the film by a whopping seven minutes. The British Board of Film Classification has allowed the film to be released untouched here with an 18 certificate. The prevailing liberal view adopted by the BBFC is to do as little snipping as possible if the sex (and/or violence) is integral to the story, and not a gratuitous addendum.

Therein lies the rub. For many of the great and good in this country the so-called art-house masterpiece has been getting away with kinky murder. Even seasoned critics can be taken aback by films that seem to crash through the boundaries of “good taste” in ways guaranteed to shock.

I was left reeling by the nine-minute rape and murder of Monica Bellucci in Gaspar Noé’s terrifying film Irréversible in 2002. The appetite for torture pornography in slasher flicks such as Eli Roth’s Hostel horrifies me almost as much as the BBFC’s refusal to censor any gratuitous part of it. But there is a profound gulf between these two films in what they are trying to achieve, and whom they are aimed at. Ir-réversibleis an extraordinary experiment in perception and time. Hostel is as morally vacant and intellectually ambitious as those famous old staples Emmanuelle(1974) and Deep Throat (1972).

More recently, 9 Songs, by Michael Winterbottom, clearly aspired to art, but the acting (its two leads were unknowns) was so bad that it became memorable for its emphasis on sex – and was an embarrassing failure as a result. The oral sex in Patrice Chéreau’s Intimacy (2001) was more memorable than the film itself.

Michael Haneke’s The Piano Teacher (2001), however, featuring Isabelle Huppert as a woman who went to porn cinemas before cutting her own vagina, was brilliant and the explicitness integral to understanding the character. In Lee’s own Brokeback Mountain (2005), the sex between the two men was glancing, though the furore it caused showed how combustible gay sex could be in the minds of a mainstream audience.

Films tend to be edited and distributed in terms of demographics: what we think is suitable for children, teenagers, and adults. This is not quite as straightforward as it seems. What’s suitable for London, New York or Paris is quite frequently regarded as the work of Satan in Bible-bashing Kansas. The result is that few ratings systems match up from one territory to the next.

Yet things are changing exponentially. Cheaper films and the internet are hastening less pedantic attitudes to censorship. There is no question that screen sex is getting far more explicit and graphic.

There’s almost a tangible aura of flexibility about the film classification in this country, and on the whole that is no bad thing. Yet no matter how radically our tastes shift, one person’s art-house sex will always be another person’s porn.

There are still vital functions for organisations such as the BBFC to fulfil, especially when it comes to filtering out child pornography, cruelty to animals and scrutinising sexual violence. There are dangers particularly with nonconsensual sex and rape depicted for entertainment value. But for better or worse movies have defined our attitudes to sex. Directors will continue to push the boundaries – and lest we think that Lust, Caution means we’ve gone to hell in a handcart, after you’ve mulled the sex scenes at the dinner party, I bet – in the end – you’ll have more to say about the sterling performances of Tony Leung and Tang Wei and Lee’s own brilliant direction. How was it for me? Pretty damn good.

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