Early in "The Savage Detectives" -- some nice sex writing in the book. E.g., too long to quote in full:

Her fingers ran over my face, from my chin to my eyes, closing my eyes as if inviting me to sleep, her hand, a bony hand, unzipped my pants and felt for my cock. Why I don't know, maybe because I was so nervous, but I said I wasn't sleepy. I know, said Maria, me either. Then everything turned into a succession of concrete acts and proper nouns and verbs, or pages from an anatomy manual scattered like flower petals, chaotically linked. I explored Maria's naked body, Maria's glorious naked body, in a contained silence, although I could have shouted, rejoicing in each corner, each smooth and interminable space I discovered. Maria was less reserved. Soon she began to moan, and her maneuvers, at first timid or restrained, became more open (I can't think of another word for it just now), as she guided my hand to places it hadn't reached, whether out of ignorance or negligence. So that was how I learned, in fewer than ten minutes, where a woman's clitoris is and how to massage or fondle or press it, always within the bounds of gentleness, of course, bounds that Maria, on the other hand, was constantly transgressing, since my cock, treated well in the first forays, soon began to suffer torments in her hands, hands that in the dark and the tangle of sheets sometimes seemed to me like the talons of a falcon or a falconess, tugging on me so hard that I was afraid they were trying to pull me right off, and at other times like Chinese dwarfs (her fingers were the fucking dwarfs!) investigating and measuring the spaces and ducts that connected my testicles to my cock and each other. Then (but first I pushed my pants down to my knees) I got on top of her and entered her.

"Don't come inside of me," said Maria.

"I'll try not to, " I said.

"What do you mean you'll try, you jerk? Don't come inside!"