The Painful Truth About Today's Pornography (warning: explicit movie descriptions) by Robert Jensen, 2004. School of Journalism at the University of Texas.

Mainstream pornography hurts women. This is sad. But maybe sadder is that there are so many people who enjoy watching people get hurt.

Robert had to describe in some detail the violence in the movies to make his point that men who watch mainstream porn get sexually aroused watching women get hurt and humiliated. He reasons that since most consumers of porn aren't interested in romantic relationships between the actors and there are a limited number of ways actors can have their bodies stimulated, there needs to be something else to keep viewers interested. Cruelty. 

Jensen didn't have to imply that the women doing porn aren't free. That's a subject for another essay.