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.