Norman Mailer has won England's annual Bad Sex Award, posthumously, for a passage in one of his last novels. Here's part of the winning passage:
...[S]he took his old battering ram  into her lips. Uncle was now as soft as a coil of excrement. She sucked on him nonetheless with an avidity that could come only from the Evil One - that she knew. From there, the impulse had come. So now they both had their heads at the wrong end, and the Evil One was there. He had never been so close before.
The Hound began to come to life. Right in her mouth. It surprised her. Alois had been so limp. But now he was a man again! His mouth lathered with her sap, he turned around and embraced her face with all the passion of his own lips and face, ready at last to grind into her with the Hound, grind it into her piety...
These characters are all related to Hitler in some way. UD's not sure what way that is, not having read the novel, and having no plans to read the novel... But UD is pretty sure -- Scathing Online Schoolmarm is pretty sure -- in what way this prose is coiled excrement.
First, although it seems to be told from the point of view of Uncle's niece ( "That she knew... It surprised her..."), a simple-minded girl (Evil Ones, Hounds... now he was a man again!), some of her language ( excrement, avidity, piety) seems to come not from her simple mind, but from the mind of a novelist ... So there is confusion on the level of characterization and diction, a confusion heightened by a combination of lots of brief phrases ( Right in her mouth. It surprised her.) and one long one (the last sentence). Brief phrases make sense if you're trying to capture a mind in the midst of sex - a simple mind, anyway - but topping things off with a long sentence ending in absurd pretentiousness ( grind it into her piety) makes a mess.
Second, there's an effort ( excrement, lathered) to be disgusting -- which, okay, make your novel's sex disgusting -- but there's no clear reason why we should be made to feel disgust. Because there's some form of incest involved? Because these are loathsome people? Does the narrator think sex intrinsically disgusting? The work of the Evil One?
And about that Evil One: Mailer's famous for putting convoluted metaphysics into his sex scenes (the notorious one-hole-then-the-other scene with the maid Ruta in An American Dream  being the touchstone here), and he seems up to the same thing in this passage, but since the metaphysics are mainly conveyed (see SOS's first point) by a simple-minded person, they are easily dismissed as primitive.
It's difficult to write about sex well - a fact amusingly exploited, year after year, by the Bad Sex Award -- and it's extremely difficult to write about sex and ideas about sex well. Many writers decide - wisely - that the only thing to do is write about sex satirically or farcically or with a sweet all-forgiving everyone's a fool sort of winsomeness. But if you, like UD and Norman Mailer, love D.H. Lawrence, you know that it's possible for the occasional genius to put sex and ideas into a novel and not be ridiculous about it. This knowledge inspires lesser writers to give it a whirl.
Which brings UD to her final point about the Bad Sex Awards. One of the short-listed passages  isn't really bad. I mean, parts of it are bad, to be sure, but it's not altogether bad, and UD wants to defend it. Especially against Andrew Sullivan's  very strong attack on it on his blog today. Sullivan thinks this passage should have won the award. But actually Mailer's is worse. I'll quote just some of the passage from this Bad Sex Award finalist, and I'll interrupt parenthetically with my own commentary:
I nearly pulled a groin muscle getting her naked, but through it all I stayed hard, a testament to how much I wanted her. [No point of view problem, as in Mailer, here. A simple, straightforward, first-person account. SOS could quibble with the writer's choice of the word testament, not only because of its religious connotations, which are out of place here, but because the word's awkwardly allied to testes, testicle...] She kept her t-shirt on throughout the initial popping [The speaker is ironically adopting his partner's use of the slang word pop], which is just how I like my sex, infused with a little mystery. [Infused doesn't work -- too fancy for the simple conversational approach the author seems to want. Mixed would be better.] I slipped my hands beneath the cotton tee and felt the smooth creamery of her breasts while saving the visuals of those brown glossy globes for later. [Too many adjectives.] Her vagina was all that, as they say in the urban media - a powerful ethnic muscle scented by bitter lemon, the breezes of the local sea, and the sweaty needs of a tiny nation trying to breed itself into a future. [This sentence begins promisingly - the ironic, self-aware, language-sensitive speaker is doing fine - but then he goes all Lawrence Durrell on us, trying to do that thing that D.H. Lawrence -- and he only sometimes -- can do -- make sex somehow convey larger human - national! - truths...] ... I find it cliched when couples insist that they have "the perfect fit," but between the busted-up, zigzag, Broadway boogie-woogie of my maligned purple khui and the all-encompassing nature of her Caspian pizda, we reached a third way, as it were. [This is, if you ask UD, funny: I find it cliched... as it were... ] That is to say, she rode me. It was all very classy and contemporary, like a modern art survey course at NYU... [See, there's a sensibility here - a raggedy one, certainly, but a sensibility... with a certain exoticism, a certain reach, a certain complexity. Who wouldn't prefer this passage, and the novel it suggests, to the doltish nonsense of Mailer?]