Why pornography works, etc, i think can best be explained at the most basic, animalistic levels of humans.
Mammals are all ingrained with the need to procreate. Visualls, I think the male inner workings are that, if they see a female being mated, or in the process of, they want to drive off the other male and be the father of the females child instead. This is pretty basic animal behaviour, and may be somewhat mirrored by human behavior. Men are usually much more aggressive around possible mates, and rather defensive when with one.
Common sexual advertisements by women are sex like movements, most commonly dancing. Pornography is a step up to where the actual movements are replaced by similar movements in intercourse.
Also i think the term is something like mirroring. When you watch something happen, it becomes much easier to see yourself in the same scenario.