Every time I watch Clerks II -and that's a lot, btw- I'm taken back to the theatre I first saw it in; the room was full of noisy cellphone-happy teenagers, but during that scene everyone was dead silent and frozen on the screen, myself included. It was a unique mixture of hilarity and creepy disbelief to see that poor kid explain why he can't have sex with his girlfriend. Jeff Anderson does an amazing job of carrying the film almost all on his own, and this scene is the perfect example: his reaction is as priceless as the puddle of pooh he's listening to.
And the cherry on top, what makes this little gem of a scene completely unforgettable, is the barely-noticeable but still effective music, ripped straight from Stanley Kubrick's The Shining. One of my top-10 favorite films last decade, and this scene here is one of the many reasons why.