How we cite our quotes:
Not rape, not quite that, but undesired nevertheless, undesired to the core. As though she had decided to go slack, die within herself for the duration, like a rabbit when the jaws of the fox close on its neck. So that everything done to her might be done, as it were, far away. (3.67)
In Disgrace, sex tends not to be a mutually desired act. Even though Melanie doesn't explicitly fight David off, he can still tell that she doesn't want it. Whether or not it can be construed as rape is something that puzzles David and also leaves us scratching our heads.
He makes love to her one more time, on the bed in his daughter's room. It is good, as good as the first time; he is beginning to learn the way her body moves. She is quick, and greedy for experience. If he does not sense in her a fully sexual appetite, that is only because she is still young. (4.1)
Pay attention to the perspective here. Who is saying that Melanie is greedy for experience? Probably not Melanie, for one. Instead, this assessment seems to come straight from David, who might be looking for ways to convince himself not only that Melanie is as invested in the experience as he is, but also that she even wants to be there in the first place. Also notice that they're having sex in Lucy's bed. This draws parallel between the two women and their sexual experiences.
Driving home from a concert that evening, he stops at a traffic light. A motorcycle throbs past, a silver Ducati bearing two figures in black. They wear helmets, but he recognizes them nevertheless. Melanie, on the pillion, sits with knees wide apart, pelvis arched. A quick shudder of lust tugs him. I have been there! he thinks. Then the motorcycle surges forward, bearing her away. (4.64)
Slow down, boy. Now, we aren't saying that the image of Melanie with her knees apart, accommodating another person's body isn't an explicitly sexual image, because it totally is. Still, we only see it because it's going through David's mind. This moment gives us another example of how Melanie gets David all hot and bothered, but from a distance. Sure, he feels the tug of lust, but he feels it as a spectator.