Sarah Churchwell: I'm sorry, but I don't like watching sex on the screen

Ang Lee's new film Lust, Caution, which opens in Britain tomorrow, is being touted as "much-anticipated". There are three reasons for all this anticipation: it is Lee's follow-up to the equally hyped Brokeback Mountain, it won the prestigious Golden Lion at the Venice Film Festival and it is, evidently, so graphically sexual that the Chinese government forced Lee to censor the film before it could be released there.

I share the anticipation for three different reasons. I am a long-time fan of Lee's work, although I was apostate about Brokeback Mountain, which seemed to me deeply implausible from the first sexual encounter (that said, I also know nothing about gay shepherd sex and thus am amenable to correction). I enjoy stories set in Shanghai, for personal and historical reasons. And I have always relished romantic thrillers, wartime espionage films and homages to them. Evidently, Lee peppers this film with tributes to some of the greatest, including the incomparable Notorious, one of the sexiest films ever made.

The one part of the film I am not looking forward to is what seems to bring everyone else through the door: the gratuitous sex. It is with some reluctance that I admit I don't particularly enjoy watching sex on film, because it makes me sound like a stereotypical Puritanical American and I detest playing to type, especially that type. I don't object to sex on screen for moral reasons on the contrary. As far as morality goes, film-makers have my cheerful permission to simulate people bonking away to their heart's content.

Nor is my objection political, although this does get trickier. I'm a bit a sceptical about easy assurances that sex on screen today is not exploitative; this seems a trifle glib. But as long as the actors aren't objecting, I'll give everyone involved the benefit of the doubt.

The truth is, I just don't want to look at it. I especially don't want to look at it when it's 10 meters high. I find sex absurd enough even when I'm actively engaged in, and enjoying, it; I don't think I ever quite recovered from the astonishment I first felt when, at the age of six, I was informed by 12-year-old Anthony from down the street of the anatomical facts of life. My response was pure scepticism: surely no one in their right mind would put that there! By amplifying the size of the genitals in question and my distance from them, sex on film simply intensifies this reaction. You want to put what where?

Film-makers have a number of tricks for dealing with the inherent comedy of watching other people have sex. The most obvious is to increase the violence; if it's scary, we will take it more seriously. Danger is not just titillating; it can also be aggrandising.

This has apparently been Lee's strategy in the case of Lust, Caution, in which the sex is evidently acrobatic, sado-masochistic, and grim. Apparently, these scenes were "difficult" to film. I'll bet. Lee has said in interviews that he found them exhausting, so just think how the actors must have felt. The crew doesn't seem to have enjoyed it, either; one crew member called filming these scenes "11 days of hell" and Lee suggested in an interview that the film itself is about hell: where Brokeback is about lost paradise, he explained, Lust, Caution is "down in the cave, a scary place. It's more like hell".

Scorching and perfervid as all that sounds, US reviewers have responded by finding the film's sex scenes "cold". And this is surely no coincidence. One does not have to feel lust in order to find a film erotic: but one does, presumably, have to feel desire. It is a clich that the censored films of the golden era which after all Lust, Caution is supposed to be emulating were crackling with suggestive tension because all the sex had to be sublimated. But the point that often gets missed is that this didn't occur only on screen; watching those films creates tension because they frustrate desire romantic, as well as sexual, desire in the moviegoers as well. Audiences can share in the characters' anticipation and frustration.

In the same interview, Lee said the sex was necessary to reveal the characters, explaining: "It's part of the plot, since it's all about acting, levels of acting. You're performing when you have sex." Well, that all depends on your relationship, but it is precisely my problem with sex on screen. It turns us all into actors, watching ourselves watching each other, which makes it a hell of a lot harder to relax and enjoy it.

The writer is a senior lecturer in American literature and culture at the University of East Anglia