Sure, Kidman and Cruise have filmed scenes for Eyes Wide Shut that were too explicit to be shown on television when they were released as publicity clips. Yes, Kidman agreed to flash her breasts and buttocks in The Blue Room. They are not, in other words, strangers to sexual exhibitionism. But even so the two may be getting into deeper waters than they realise. For what, these days, can possibly qualify as a sufficiently realistic love scene?
Once upon a time, say about 50 years ago, when Alfred Hitchcock made Notorious, it was sufficiently realistic for Ingrid Bergman to press her mouth against Cary Grant's while the circling of the camera suggested the welling of a vertiginous desire. Even about 25 years ago, when films had become a whole lot muckier, and Martin Scorsese made Taxi Driver, it was still enough for all the dangerous sex to be contained in conversation. "You can come on her, you can fuck her in the mouth," detailed the pimp, and although audiences might have recoiled with the shock of the graphic language, all Jodie Foster actually had to do was to look miserable.
Even a few years ago mainstream films hadn't gone much further; nudity was mandatory for female stars. However, directors still had a peekaboo attitude to sex. In the supposedly sexy Basic Instinct, "sufficiently realistic" sex scenes involved - as Sharon Stone put it scornfully - "anatomically impossible positions" designed to show off the leading actress's body rather than suggest anything as rude as intercourse.
However, over the last year or so, a sufficiently realistic love scene has become something very different, and it's become almost real. This greater explicitness has crept up on us without fanfare and without a backlash. Instead, there has been a fascinating silence while the shift has taken place.
And this is a real shift, make no mistake about that. A film that shows close-up shots of penetrative sex, Lars Von Trier's The Idiots, has been passed for cinema exhibition in Britain without any outcry. Visual jokes that involve ejaculation have suddenly become commonplace - even if you didn't see There's Something About Mary you knew that the film involved a gag in which Cameron Diaz slicks back her fringe with her boyfriend's semen. But hardly anyone has bothered to warn cinema audiences that Todd Solondz's new film, Happiness, has two similar scenes. In the first, the masturbator sticks a Post-It note to his wall with his semen; in the second the camera lingers lovingly on the drops clinging to the balcony wall before a dog walks into shot and licks them up. When I saw the film, a mere ripple of bored laughter floated around the cinema.
And men are now more often finding themselves cast as explicit objects of desire - the most vivid sex scenes ever seen in television drama were those in the recent, surprisingly sexy Channel 4 series Queer as Folk. Without warning, that series slid from frank conversation into shots of tongues in buttocks, hands on crotches and just-out-of-frame oral sex.
The greatest barrier to showing explicit sex on our screens has always been the censors' inexplicable unease with erect penises. Yet even that is now breaking down. The Idiots has a lingering shot of a penis becoming gradually erect.
This new ease in portraying the physical reality of sex in mainstream film and television may well have a positive side. It certainly makes it possible to imagine a more equal portrayal of sexuality, in which men's bodies as well as women's take the weight of the camera's interest. And it makes it possible to imagine that less ignorance might surround the realities of sex, and more acceptance that sex isn't always a dream-like ballet carried out by young models.
But if I were Cruise or Kidman, I don't think that I'd be trumpeting my ability to do real sex on screen. Instead I might be worried. For generations, the glamour of Hollywood has been predicated on its distance from real life, its violence without pain, its roses without thorns. If it's going to turn itself from a dream factory to a fly-on-the-wall, how long will the bubble of cinematic celebrity last? When Cruise and Kidman finally get around to having sex for real on screen, how many people will stick around to watch the show?