Twilight likes Fluttershy, and after reading a romantic novel, she wants to tell her through a letter. When she runs into Rainbow and Pinkie, she asks the two to help her with the definitions of some of the words so she can incorporate them in her letter. She forgot one thing though: It's Sunday, which is the day Rainbow and Pinkie play their pranks, so when they hear the words she needs defined, they decide to have a little fun with the silly, innocent Twilight.
Sex tag for words relating to sex and also implied sex.
Note: While writing this, I realized that this plot is very similar to another story called Crude Misconceptions. I talked with the author and explained that I didn't realize there was a similar story, and they kindly allowed me to continue writing mine, so please check out their version