He rolled onto his side, eyes fixed and unseeing, and indulged a cinema fantasy; she pounded against his lapels before yielding with a little sob to the safe enclosure of his arms and letting herself be kissed; she didn't forgive him, she simply gave up. He watched this several times before he returned to what was real; she was angry with him […] (1.8.4)
There's a contrast here between Cecilia yielding, which is false, and Cecilia angry, which is true. But… we find out later that Cecilia isn't really angry, even if she thinks she is, and we see her yielding. With sex, as with everything else in the novel, truth and fiction are hard to pin down.
He had been about to conjure for her a private moment of exuberance, a passing impatience with convention, a memory of reading the Orioli edition of Lady Chatterly's Lover, which he had bought under the counter in Soho. (1.11.50)
Lady Chatterly's Lover by D.H. Lawrence is a famously dirty book, and the Orioli edition was the uncensored version. The novel is giving a shout out to some of the most famous sex scenes in literature before getting on with its own (sole) steamy scene.
His experience was limited and he knew only at second hand that they need not lie down. As for her, beyond all the films she had seen, and all the novels and lyrical poems she had read, she had no experience at all. Despite these limitations, it did not surprise them how clearly they knew their own needs. (1.11.65)
Up until now, we've mostly seen people thinking about sex by reading or imagining—always through second-hand encounters. Here, though, the novel suggests that those second-hand versions of sex are pretty second-rate. They don't actually help Cecilia and Robbie much at all, but at the same time, they sort things out just fine. There's some irony here, though, since this scene is itself a scene within a book (within a book). If Cecilia had somehow read it before her library rendezvous with Robbie, would it have provided useful information?
A drop of water on her upper arm. Wet. An embroidered flower, a simple daisy, sewn between the cups of her bra. Her breasts wide apart and small. On her back a mole half covered by a strap. When she climbed out of the pond, a glimpse of the triangular darkness her knickers were supposed to conceal. Wet. He saw it, he made himself see it again. The way her pelvic bones stretched the material clear of her skin, the deep curve of her waist, her startling whiteness. (1.8.2)
This is Robbie remembering Cecilia coming out of the fountain in her underwear. It's important that we see his perspective in retrospect; that is, we don't hear his thoughts at the time, but only later. Cecilia becomes an image, or a character, in his mind. Sex is about stories as much as acts (which is why the act is set in the library, perhaps).
Cecilia felt a pleasant sinking sensation in her stomach as she contemplated how deliciously self-destructive it would be, almost erotic, to be married to a man so nearly handsome, so hugely rich, so unfathomably stupid. He would fill her with his big-faced children, all of them loud, boneheaded boys with a passion for guns and football and aeroplanes. (1.4.36)
We never actually hear Cecilia express sexual desire for Robbie—the library scene is all from his perspective, not hers. That means the only person we hear her kind-of sort-of lust after is Paul, who she thinks is so awful that it would be "almost erotic" to marry him. Maybe this "deliciously self-destructive" impulse is what draws Lola to Paul as well?
"How appalling for you. The man's a maniac."
A maniac. The word had refinement, and the weight of medical diagnosis. All these years she had known him and that was what he had been. (1.10.34-35)
Lola labels Robbie a maniac just after she's been assaulted by the actual maniac, Paul. Briony is excited to have the right word. She loves words, remember. You wonder if she accused Robbie just because she's so excited to have her vocabulary confirmed.
"Bite it," he said softly. "You've got to bite it."
It cracked loudly as it yielded to her unblemished incisors, and there was revealed the white edge of the sugar shell, and the dark chocolate beneath it. It was then they heard a woman calling up the stairs from the floor below [...]
Lola was laughing through her mouthful of Amo. "There's Betty looking for you. Bathtime! Run along now. Run along." (1.5.74-76)
Paul urges Lola to eat his chocolate bar. It's not stated explicitly, but this is probably the moment before Paul's first attack on Lola. Remember from the discussion of chocolate in the "Symbols" section that Amo is linked to both violence and sex or love. Paul urging Lola to eat the chocolate is the closest we get to seeing the rape. It's one of the queasiest scenes in the book (and also one of the queasiest scenes in the 2007 film).
Then, after a few moments' reverie, tilted back on his chair, during which time he thought about the page at which his Anatomy tended to fall open these days, he dropped forward and typed before he could stop himself, "In my dreams I kiss your cunt, you sweet, wet cunt. In my thoughts I make love to you all day long." (1.8.15)
The implication is that his Anatomy keeps opening to the page showing the female genitalia. Sex for Robbie is self-reflective; he thinks about it through books and words and dreams. Is sex self-reflective for Paul? It doesn't seem to be. The thing that is supposed to make Robbie a sex maniac, then—the fact that he thinks about sex—could be seen as the exact thing that makes him not a maniac.
In love with her, willing himself to stay sane for her, he was naturally in love with her words […] During his time inside, the only female visitor he was permitted was his mother. In case he was inflamed, they said […] He had been diagnosed, with clinical precision, as morbidly oversexed, and in need of help as well as correction. He was not to be stimulated. Some letters—both his and hers—were confiscated for some timid expression of affection. So they wrote about literature, and used characters as codes […] Mention of "a quiet corner in a library" was a code for sexual ecstasy. (2.80)
Robbie is in love with Cecilia's words. At the same time, the library where they had sex among the books becomes a literary code for sex. Who knew reading a novel could be so exciting?
Poor vain and vulnerable Lola, with the pearl-studded choker and the rosewater scent, who longed to throw off the last restraints of childhood, who saved herself from humiliation by falling in love, or persuading herself she had, and who could not believe her luck when Briony insisted on doing the talking and blaming. And what luck that was for Lola—barely more than a child, prized open and taken—to marry her rapist. (3.260)
We mentioned this quote in the "Coming of Age" theme as well. Sex and coming of age are often connected. Usually, having sex is seen as one way that people grow up, and there's some of that in this novel too (Robbie and Cecilia both feel transformed by their sexual encounter, for instance). But this quote suggests that the two can be linked in other ways too. Specifically, wanting to grow up makes Lola confused about both sex and love. Sex doesn't make her an adult. Rather, wanting to be an adult, or being caught between adulthood and childhood, makes her unable to see Paul's thuggishness as thuggishness. In some ways, Paul's rape traps her as a child forever. Because of his assault, she ends up living her life based on a crush she convinced herself she had when she was fifteen.