The story is from the point of view of a male. He has two friends that come back from college (I think). A guy and a girl and they decide to sneak into their neighbors pool. They dare each other to undress which they do. The neighbors then come home and the main characters buddy is able to book it out of there, while the main character and the girl are stuck hiding in a small shed. They spy on the neighbors as the neighbors start having sex in the pool. The main character and the girl become aroused and as a result of their cramped, sweaty situation, they “accidentally” have sex.
The story and the relationship then develop from there.
Source: reddit.com/r/eroticliterature/comments/4nxcbm/looking_for_a_particular_sex_story_between_two