Once, we even watched the movie

The Odeon Birmingham recently toyed with the idea of banning kissing in the auditorium. They'll be banning films next.

It is one of the abiding clichés of cinema. A young couple sit side by side, their faces dappled with moving light, their eyes fixed firmly forwards. And then the boy's hand lifts from his lap in a gesture of studied insouciance - for a casual scratch of the ear, perhaps, or an unthinking flick of the hair. Instead of descending again, though, it hovers, before twisting awkwardly to land on the seat back. And then, while both boy and girl continue to stare resolutely face-front, it inches sideways, flaps briefly in space like a panicked bird and then flops heavily on the girl's opposite shoulder. And what's unusual about this particular cliché, unlike most of those we experience at the movies, is that it exists on both sides of the screen. Out there in the darkness, real couples are probably going through the same complex docking manoeuvres, anxiously trying to convert mere proximity into genuine intimacy.

It is one of the abiding clichés of cinema. A young couple sit side by side, their faces dappled with moving light, their eyes fixed firmly forwards. And then the boy's hand lifts from his lap in a gesture of studied insouciance - for a casual scratch of the ear, perhaps, or an unthinking flick of the hair. Instead of descending again, though, it hovers, before twisting awkwardly to land on the seat back. And then, while both boy and girl continue to stare resolutely face-front, it inches sideways, flaps briefly in space like a panicked bird and then flops heavily on the girl's opposite shoulder. And what's unusual about this particular cliché, unlike most of those we experience at the movies, is that it exists on both sides of the screen. Out there in the darkness, real couples are probably going through the same complex docking manoeuvres, anxiously trying to convert mere proximity into genuine intimacy.

Not for much longer in Birmingham, apparently, or at least not in the Odeon multiplex, where the manager is reported to be considering a ban on over-enthusiastic back-seat snogging. After a brief intermission for general disbelief the Odeon chain uttered some conciliatory noises to potential snoggers and snoggees - "We have had 70 years of people coming in and snogging in the back row," said a spokesman, "and long may it continue." It was a wise retreat, since a cinema without snogging is like Henley without the Pimm's or strawberries without cream. The Odeon's own current mission statement - "Fanatical about film" - doesn't quite acknowledge the blurry promise of sex and speculation that has always been one of the main attractions of the movies, and which has long found physical expression in the shape of courting seats - a row of double-width chairs that offer entwined couples some relief from painful armrest interruptions. When the Prince Charles cinema in London recently refurbished its ancient seating it kept its courting seats for reasons of structural nostalgia - but Jacqueline Swain, the cinema's box-office manager, admits that they have little practical purpose now. "In a cinema like ours, most people come to watch the films," she says. It's almost as if a faint air of confessed perversity hangs over the remark. Going to a cinema to watch films? Well, it takes all sorts.

The Prince Charles's clients, largely enthusiasts and film buffs, should certainly know the theory even if they don't put it into practice, because if you want to know why cinemas are sexy places, films are the very first place to look. When the screen chooses to hold up a mirror to the space in front of it, the reflection is not just flattering, which you might expect, but it's also charged with erotic possibility. On film, television watchers are always portrayed as torpid and stupefied - drugged by the blue light into a state of detumescent isolation. Movement of any kind would be startling, let alone a sexual move. Film audiences, by contrast, are almost always shown in a state of excitation or arousal - even if the excitement isn't always generated by what they are notionally supposed to be looking at. In film after film the auditorium is shown as a magical space where regulation is relaxed and all kinds of forbidden contact become possible.

On occasions this can be crudely literal. Cinemas are dark places and naturally lend themselves to furtive explorations and "accidental" collisions. Barry Levinson's autobiographical film Diner includes a memorable scene in which a young man, having made a wager with his friends that he can get a prudish local beauty to touch his penis before the end credits, wins his bet by putting a large carton of popcorn on his lap and adulterating its contents from beneath. Proof of his achievement comes when the rustle of her innocently foraging hand finally gives way to a loud shriek of alarm.

A rather more subtle exploitation of the possibilities occurs in Kubrick's Lolita, when Humbert Humbert goes to the movies with Lolita and her mother, and wandering hands quickly lose their bearings in the dark. It's a funny scene, in which very different kinds of desire find themselves unwittingly tangled together, and it reminds us of another straightforward reason why the cinema should be a place of erotic possibility - because it squeezes people together into an unaccustomed closeness. There can't be many people who haven't experienced the tingle of an adjacent arm, so near and yet so far, or fretted about the etiquette of those inadequate armrests - are they like side-plates, aligned to one side or the other, or are they designed for sharing? Whatever the case, a pair of tickets for the stalls is still among the least fraught ways for two people to contrive to approach a little closer and still have an escape route.

But if cinemas only provided low lighting and proximity they could hardly have achieved their almost dream-like status as places of romantic assignation. It isn't just that they offer an unimpeachable retreat from parental supervision or a place where social rules are looser (when Celia Johnson meets Trevor Howard in Brief Encounter, it is after her weekly trip to the cinema - a solitary pleasure that can arouse no anxiety in her trusting husband). It's that they are places of powerful suggestion. Cinema can be relied on to raise the unmentionable subjects - love and sexual attraction - but its influence usually goes further than that.

In Larry McMurtry's The Last Picture Show, there's a perfect description of the triangulated fantasies that are possible when real and fictional sex brush up against each other. Sonny has gone to the movies with Charlene and, after a slow start, the two settle down for a proper back-row necking session, one in which both their attentions are divided. "It seemed to her that Sonny looked a little bit like Steve Cochran, and she began to kiss him energetically, squirming and pressing herself against his knee. Sonny returned the kiss, with somewhat muted interest. He wanted to keep at least one eye on the screen, so if Ginger Rogers decided to take her clothes off he wouldn't miss it. The posters outside indicated she at least got down to her slip at one point."

This dangerous power of cinema to arouse its viewers has long been recognised by moral policemen. In Cinema Paradiso the village priest previews all attractions, ringing a bell when he decides that a scene might endanger the spiritual welfare of his parishioners, so that the projectionist can snip it out (in practice this means any scene in which less than six inches separate a man and a woman). Despite this butchery the village cinema remains a place of carnal vitality - a place where the audience finds itself emotionally lifted out of its isolation while surrendering none of its close-knit familiarity. For the lovers in that audience, screen sex is not a necessary accessory to their own fantasies, just a useful one. This isn't always the case, as Sonny finds out to his cost: "at first he had thought Charlene's fits of cinematic passion very encouraging, until he discovered it was practically impossible to get her worked up except in picture shows."

Charlene is obviously completely dependent on the aphrodisiac of celluloid, but few of us are entirely immune to it - because the creatures we stare up at are contrived so carefully to stir romantic longings. One of the most beautiful accounts of this can be found in Woody Allen's film The Purple Rose of Cairo, in which Mia Farrow's lonely, abused wife falls in love with a screen idol, who eventually descends from the screen to be with her. Allen's comedy is at its wisest when it recognises the inadequacy of screen perfection - Jeff Daniel's hero is marvellous at kissing but can't go any further, since his Thirties character never gets beyond an innocent smooch. This isn't quite the problem it once was, of course, and it isn't a problem at all in porn cinemas, which have always been frenzied locations for sexual activity, solitary and otherwise. But a broad truth remains in conventional cinema - sex on the screen is never truly consummated; the fantasy must be resolved, one way or another, by us. We have to finish what they have started. Trying to stamp out sex in the cinema is an almost comically pointless task because you can't shine an usherette's torch inside someone's head - and that's where the really steamy stuff is taking place.