Thinking about how mirror neurons work makes thinking about pornography and voyeurism all the more interesting, if not confusing. For any of you not familiar with our mirror neurons and how they function, have a look at a very good short little intro video from Nova Science Now http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html
Basically, when we watch someone else do something, our brain responds almost as if we are doing what we're watching -- we watch a dancer and our motor neurons fire, we watch someone smile or frown and our brain responds as if we're happy and smiling or frowning and fuming. Watching others is the way we learn how to function and behave in our culture, and, because we feel what others feel when we watch them, mirror neurons are also central to having empathy for others.