Quote 1
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.
Quote 2
"I was not myself. I was no longer a fifty-year-old divorcé at a loose end. I became a servant of Eros." (6.63)
Here, David explains to the committee just why he was so bewitched by Melanie. She inspired passion in him. At first it seems like he's giving himself a way to argue temporary insanity ("I was not myself, your honor"), but we know from his guilty plea that David isn't really out there to make excuses. Instead, he's confessing that Melanie had the ability to totally transform his sense of love and passion. Still, he blames it on Eros (Cupid) instead of taking personal responsibility for what he did.
Quote 3
He wonders how it is for Lucy with her lovers, how it is for her lovers with her. He has never been afraid to follow a thought down its winding track, and he is not afraid now. Has he fathered a woman of passion? What can she draw on, what not, in the realm of the senses? Are he and she capable of talking about that too? (9.12)
Thinking about sex is second nature for David, and even though it seems he hasn't thought about Lucy's sex life in the past, he nevertheless isn't weirded out by wondering about her experience. What is interesting here is that it seems that David genuinely wants to talk with Lucy about sex; he's interested in what her experiences have been like. Is it possible that he wants to compare and contrast to see if other people feel the same way he does about sex?