I heard on NPR this morning about a new study which shows that male chimpanzees enjoy looking at photographs of naked female chimpanzees. So much so, in fact, that they are willing to give up their food in exchange for a chance to view the photographs.

So not only is it a darwinian imperative to want to look at porn, it's evolutionarily hard-coded that we will pay for porn. Only (in some of our cases) does our highly sophisticated sense of cheapness counteract this urge.