If ever a couple locked lips on screen – regardless of how much grunting there was or how many buckets of saliva exchanged – my granny used to hold her hands in front of her face and grimace. She did it even when the couple were fully-clothed and just saying a quick goodbye.
"Ugh, tell me when it's over," she'd ask of my family, all of whom were pinned to the sofa in mortification, trying not to let any muscles spasm in case anyone thought we were aroused.
"There's a certain amount of embarrassment on the part of the audience," says Anthony Quinn, i's film critic. "And generally sex scenes in films are just completely unsexy.
"The real challenge is to make a film sexy without people tearing their clothes off." My granny's discomfort wasn't born of a Mary Whitehouse complex or generational prudishness – she just loathed the visceral nature of it all: the bodily messes and slurps, the sweaty humping.
It isn't an unreasonable reaction.
How times have changed since the first on-film French kiss between Warren Beatty and Natalie Wood in Splendor in the Grass. That snog celebrates its 50th birthday this year, but it's nowhere near as grown-up as what's currently going on at the cinema. This year alone we've had extensive oral sex and lesbian kissing.
Blue Valentine, starring Ryan Gosling and Michelle Williams, was released in the US at the end of last year and follows the, ahem, ins and outs of a couple's courtship, marriage and subsequent separation. Originally rated NC-17, the filmmakers had this overturned and the film, which was released here last Friday, has a UK certificate of 15. The controversy centred on a prolonged and highly intense oral sex scene that has made audiences the world over squirm and look away. "I went to see it with my wife," remembers Anthony Quinn, "and she turned round to me with a face like Queen Victoria, her mouth was puckered in disgust." Thank goodness my granny isn't around to see it. Meanwhile, Natalie Portman's latest film Black Swan, which opens on Friday, tells the story of a young ballerina's descent into near-psychosis as the pressure of it all gets to her; the culmination of all this is – of course – a raunchy bit of girl-on-girl action; of a similar variety to Blue Valentine and, also notably, 2010's The Kids Are Alright. Three's a trend, as they say.
"Sex scenes are responsible for many people getting very dissatisfied with their sex life," argues sex expert Tracey Cox. "They're radically different from real life and they make people compare. And when it's not perfect, we're a bit shocked."
Every so often there's speculation about the veracity of the on-screen writhing, of course – in some films, it comes across as unbearably choreographed, with the principal boy practically counting every thrust before flipping his partner over as part a Pineapple Studios-worthy routine. "In the film-makers' minds, of course, it's supposed to be sexy," says Quinn, "but I just find it mortifying seeing people pretending to have sex and I think it's counter-productive in terms ofaudience enjoyment."
But films like Don't Look Now and The Postman Always Rings Twice have gone down (snigger) in history as some of the steamiest celluloid ever created.
Did they? Didn't they? Can you see it? Can't you?
The point, of course, about such scenes in mainstream films is not how much you can see, but the very fact that the bits you can't see are the only things you can think about. We're bombarded with breasts and bottoms and occasionally a little bit more. We're the generation that grew up with internet porn (well, some of us).
But have we become completely inured to the presentation of sex on the screen? The rudest of the rude no longer hits the headlines for being lewd or unnecessary; your local vicar isn't about to issue an edict against going to watch them: sex in films has become par for the course.
"In the Eighties, there was far more nudity," says Tracey Cox. "Things were raw and bodies were more real. Now we're beauty-obsessed; scenes are beautifully lit, carefully orchestrated and quite sterile."
It brings to mind Martin Freeman in Love Actually, playing a porn-movie extra, naked as a baby above Joanna Page, trying to muster the courage to actually ask her out in real life.
It speaks of some latent humanity, perhaps, that, despite our porn culture and beauty myths, we're not interested in sex simply for titillation at the cinema – it just doesn't work that way.
The latest example is teen mecha-franchise Twilight, the final part of which is released later this year. Stills from the much-awaited sex scene between Bella and Edward, who insist on waiting until they're married, last week leaked on the internet. There are no dangly bits, no gurns, no sweat; just eye-gazing and milkily inoffensive coloured shoulders. My granny might even have been able to stomach it.