I always wondered why a sexual comedy film would be called American Pie. I mean what is the significance of "Pie" in the title?
This is answered pretty well on the Wikipedia article. Specifically, the last sentence in the quote below.
The title is borrowed from the folk song of the same name and refers to a scene in the film, in which the lead character is caught masturbating with a pie after being told that third base feels like "warm apple pie". It's also been stated by writer Adam Herz that the title also refers to the quest of losing your virginity in high school, which is as "American as apple pie."
The writer is using "pie" as a metaphor for your first sexual experience. When you read the title "American Pie," don't think of it as a delicious desert, but rather as a sexual innuendo.