Martin Amis correctly observes that sex scenes in novels are uniformly terrible. There is a Bad Sex Award given out each year by the Literary Review, but no good one. There is a good reason for this, which can be observed in the simple sentence:
He got into his car and drove off.
That sentence masks a hundred little actions. Were it fully written, it would go something like:
He reached down and grasped the cold metal of the door handle that sat in the bright red paint of the car's side, with one firm tug it moved upwards and with that movement the door came open allowing him to climb into the L-shape of upholstery that was the driver's seat... etc etc etc he pressed his left foot down onto the clutch that gave way and, with his left hand he reached for the gear lever... etc etc etc
'Yes,' thinks the reader, 'I know. That's how you get into a car and drive off.' A standard action requires no narration. It's nothing to do with whether the language is flowery or plain, good or bad. We do not describe the process of breathing in and out, nor the mechanics of opening the fridge. And sex is, unless you're the Marquis De Sade, a pretty standard business.
In fact I should add a caveat at this point that if there is something astonishing about the sex then the novelist should of course describe it. But what is astonishing is never, or rarely, the sex.
He got into his Batman costume and she started reciting A Season In Hell, and they had sex.
Is enough. The sex itself is going to be a mechanical business. From the reader's point of view, it is everything else that is interesting: the lead-up, the aftermath, the wallpaper in the bedroom, all is going to be more interesting than the actual physical fandango.
They had sex beneath the red patterned wallpaper. She giggled throughout.
Anything but elaborate on that one phrasal verb.
Of course, it's a terribly emotional business, sex, or so I've been told. And when something is terribly emotional it seems important. But I don't think that's the case. People get emotional about all sorts of things. Flags, for example, people get terribly emotional about their country's flag. But that does not mean that the Union Jack requires description. We know what it is, and that is that.
Their mother had told them that if they had dirty thoughts they'd turn to stone, but they didn't listen.