This was after the controversy over Beavis always saying, "FIRE FIRE!!" and parents freaking out because they were afraid their kids would burn the house down. So the fact that he's wide-eyed and sex-moaning is a tribute to his old ways.
I've just realized that the movie was likely made after that point. There's a point toward the end of the movie where Beavis says the line, which would therefore have been massive fanservice. The fans would have gone wild... yelling 'fire' in a crowded theater. Oh god. The LAYERS.