I can't remember the author or name of the story, but I do remember the general plot.
Female MC is a teacher and fantasizes about some of her students. Then she is participating in the school fair at a stand where she sticks her head through a hole in a wall and let's the students try to hit her with a cream pie or water balloon or something. While she is there, her boyfriend (fiance/husband?) starts feeling her up behind the wall hidden from observers, then starts to have sex with her while she attempts not to show a reaction. She is very close to cumming when she sees her boyfriend in the crowd at the front of her booth and realizes that he isn't the one fucking her.