because: "Does the pope shit in the woods" just isn't offensive enough
This is a sarcastic answer to a question with an obvious answer of "yes." Taken as a combination of the similarly-intended ones, "Does a bear shit in the woods?" and "Is the Pope Catholic?"
Roast Beef: Um...do you want to hit up some nachos and watch some Braveheart?
Ray: Does the pope know sluts?