Why are humans excited to watch pornography?

I know that some do it while masturbating as a visual aid so they obtain pleasure. Though this is somewhat in opposition to natural selection as you're basically watching a rival. From what I know humans are the only creatures that do that. Is it a case of: reason triumphs over instincts (I mean that the brain goes over the basic instincs and overrides them). It has something to do with social behaviour?