They are supposed to bring a frisson of excitement to an otherwise pedestrian movie - but bad sex scenes only make us roll around with laughter, it seems.
Now a Top Ten of the most embarrassing couplings ever to make it to the big screen has been compiled by critics.
Pride of place goes to Kyle MacLachlan and Elizabeth Berkley, whose frenzied swimming pool romp in the big-budget flop Showgirls from 1995, was voted the biggest letdown of all.
'It is supposed to be the best sex in the world but, as Berkley thrashes around in the water, it looks more like the first ten minutes of Jaws,' said Empire magazine, which compiled the list.
Damage, the European art-house movie starring Jeremy Irons and Juliette Binoche, was placed second for its series of trysts which climax in a bizarre scene where Irons expresses his ecstasy by banging his lover's head on the floor.
'How such quality people cooked up such a farrago is a mystery to rival the Bermuda Triangle,' is Empire's verdict.
Another star branded a bedroom flop is Madonna, who dripped candle wax on Willem Dafoe in Body Of Evidence. And Sharon Stone, who the magazine calls 'the Queen of crap movie sex', makes sixth on the list with The Specialist.
Of ex-lovers Ben Affleck and Jennifer Lopez, at No.7 for Gigli, Empire says: 'Unfeasibly bad, the pair, while sexy as hell on their own, together could not generate enough heat to toast bread.'
At No.9 are Keanu Reeves and Carrie-Ann Moss for their 'damp squib' union at an underground rave in Matrix Reloaded.
