This is mine. This is from Tanith Lee's Sung in Shadow, a retelling of Romeo & Juliet. The characters involved are Iuletta's mother and Leopardo, her nephew (the one with the ongoing feud with Romulan). I like that entire scene, the brawling leading up to the act itself, but here's the culmination:

At that first thrust, she gave a tiny indrawn cry. He had no regard for her anticipation, had never troubled with such niceties, and now, his lust almost murderous, he had no margin or wish to learn. But she, who had been raped and used and left to freeze for so many years, had found in these courtships of words and blows the stimulus no rough and no cunning caress could provide. Leopardo was food to her, food, drink, religion, life. At the first surge of his body into hers, her own body surged and came to quickness. Touching him, clasping him, feeling the quivering tension that ran through and through and through him, her own flesh was educated, copying his. Her response was so great that, his attention crazily attracted, he raised his head, gasping, to look at her. And so for the first time in his selfish impoverished existence, beheld the essential duality of eroticism under him, twisting and straining and striving as he himself twisted and strained and strove. And seeing this, his eyes blackened and his heart engulfed him and he fell down on her into the great explosion of ecstasy, vaguely astonished to hear his voice cry out just as hers did. Only as spasms of pleasure withered and he came back to himself did he grasp the inevitable significance of it. Explicit sin was sweeter than mere carnality.

Now I want to read your favorite.