Scenes where they drop the premise once the sex begins

[lololulu19]

There was a good movie in which two stepbrothers that annoy each other are coaxed into sex. While the scene is good, it could have been much better very easily if they maintained the premise of the scene throughout the entire scene.
In almost all scenes that begin with a premise, the premise is diminished or gone after the sex starts.

I just uploaded a parody of "Snow White & the Seven Dwarves" in which the characters keep their costumes on (except for exposing their cocks and asses) throughout the scene.