Legend has it that Julie Christie and Donald Sutherland really DID do it in Nicholass Roeg's massively-scary 'Don't Look Now', though neither actors or the director have (to my knowledge) ever confirmed the fact. But it does look incredibly realistic.
But I recently discovered an even more convincing sex-on-the-screen scene. It's in the French film 'La Maitresse' which stars a young and extremely hunky Gerard Depardieu with the divine Bulle Ogier, who plays a dominatrix. There's a scene in the movie where Depardieu is driving an open-topped sports car with Bulle Ogier sitting on his lap. And by the looks on both their faces they're clearly not simulating intercorse! You'll find the clip on YouTube. Does anyone else agree - or is it my imagination?