"Grey" by Lysander. In proportion to its overall length, there's actually relatively little sex in this story. Grey is the name of a woman to whom the narrator offers a ride. Throughout the first half of the story they drive across the country for several weeks, longing for each other, but not making their mutual feelings known. Finally, they do make their feelings known to each other, and the sexual activity becomes loving and hot. There are several complications that make the story more interesting, but you can find these for yourself. I really like stories that have good character development as well as hot sex, and this is one of those stories.