Ok...I don't understand this pudding thing...but I'm going to assume it's something disgusting?
For those who are sophomoric in their loathing of all things remotely male homosexual, the "chocolate pudding" is meant to invoke an image of post oral-anal activity. Just like Monica testified she did to President Clinton.
Later, the camera pans to the movie theatre's "Now Playing" list, and the currently playing movie is titled: "A Bunch of Gay Cowboys Eating Pudding."
That's why when they announced the plot for this movie on FR, a lot of posters asked, "Will they be eating pudding?"