Kung Fu Monkey on writing
ETA: Will Shetterly on the above:
Sex and violence are inherently more interesting in ensemble stories, because we can't be sure that a character or a relationship will survive.
(word to the wise: swallow that coffee before you click that link.)
Or to boil it down a little more succinctly (because they're talking about the same things there)--sex and action aren't enough to carry a scene. Neither is anything else external. Not even big explosions (I'm looking at you, George Lucas.)
What carries a scene is tension, escalation, revelation, and resolution. In other words, if you're going to bother to show that sex scene or that fight scene, something needs to happen other than sex or fighting. Something has to change. The plot has to advance. The characters have to grow.
Which is why the sex scenes in A History of Violence are so effective, by the way. Also, the ones in The Cooler. They're Story, not Sex. (Although I admit, I never wanted to know that much about William H. Macy's anatomy, and I will carry the scars to my grave... but I honor him for his courage. *g*)
There also needs to be detail and a reason to ground yourself in what you're reading. The detail Friedman mentions there--the skin reddening under a hand--is the difference between effective writing and... alt.sex.stories.moderated as a drinking game.
Just like writing setting, or dialogue, or whatever; it needs to do more than one thing. Reveal character, build world (aid suspension of disbelief, if you aren't in a worldbuilding genre), create or resolve tension, advance the story. Ideally, all of the above.
Otherwise, you're killing trees for no reason.