The release today of the movie Puffball is timed to honour the impending 80th birthday of its distinguished director, Nicholas Roeg. But the screening date also marks another cinematic anniversary: it's exactly 35 years since Roeg's masterpiece, Don't Look Now, introduced what remains one of the most celebrated movie sex scenes: an extended, fragmented, ecstatic encounter between a naked Julie Christie and Donald Sutherland.

Don't Look Now and Puffball are both adapted from stories by novelists resident in the West Country - Daphne du Maurier and Fay Weldon - but are also connected by a desire to show sex in surprising ways. Whereas the earlier film was pioneering in the length and intensity of its bedroom scene, the couplings in Puffball are notable for their level of biological candour. Cutaway shots show semen shooting towards a womb, an internal shot of a penis entering a vagina and, externally, the removal and discarding of a condom. Indeed, the abandoned prophylactic goes beyond a prop to become almost a character, this most disposable of items surviving to appear in several scenes.

These moments feel strongly like an attempt by the old Roeg to give the sex scenes the kind of impact he achieved three-and-a-half decades ago. But the journey between the two films shows the change in the relationship between moviegoers and erotic material.

There has always been a close relationship between cinema and sex. In more decorous decades it was the only place that unmarried couples could hope to grope in the dark. And, even as picturehouses lost their significance as a place to practise, they increased in importance as a lecture hall for theory.

The early 70s was the period in which cinema pioneered soft-porn: films such as Last Tango in Paris, Emanuelle and Bilitis challenged censors and local authorities to admit to mainstream cinemas scenes of a kind that had previously been restricted to basements subject to busting by vice squads.

But, although Don't Look Now was inevitably clamped with an X-certificate in those sensitive times, what's most interesting about the Christie-Sutherland scene is the extent to which it stays within the limits of the licit, both visually and in context. Part of the remarkable charge of the sequence is that movie screwing is almost always adulterous, but Don't Look Now showcases marital lovemaking. And the arty composition - using reflections, quick cuts and a time-slip trick in which the sex is interleaved with the couple dressing afterwards - would make it frustrating for anyone hoping to use the movie as a masturbatory aid. In an age of liberation, the director still respects limitation.

In the decades between Don't Look Now and Puffball, cinematic censorship has progressively relaxed until, recently, actual sex acts have featured in the mainstream movies Nine Songs and Intimacy. The gynaecological moments in Puffball are presumably Roeg's contribution to this trend. Interestingly, though, perhaps reflecting the generation of filmmakers to which Roeg belongs, the attitude to nudity remains notably old-fashioned.

Kelly Reilly's naked breasts are prominently shown, but the penises of the two men who feature in sex scenes are well-protected, except for the phallus in the interior penetration scene, which presumably isn't the actor's, unless Roeg had an unusually accommodating cast and very swanky cameras.

He has, though, come to the party too late. An almost daily lesson of our culture is the way in which the internet has transformed our lives, to an extent which we have yet fully to assimilate. Among the biggest victims of this competition has been the erotic power of cinema. When Don't Look Now was released, the big screen was the only place that people might expect to see sex scenes in which they were not personally involved. Those requiring more explicit images had to risk the less lit parts of cities or mail order firms on the continent.

Now, any act involving any actors - animal, child, living, dead - is available online. Philip Larkin, a poet subsequently revealed to be keen on porn, immortalised the view that "sexual intercourse began in 1963". But, for cinema at least, sexual intercourse ended in about 2005, when the most explicit images possible became as readily available as television.

The career of Nicholas Roeg, from Don't Look Now to Puffball, is a perfect graph of this process of sexual desensitisation. In a society with the slogan Look Now, cinema is no longer the place that people have to go for sex.



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