| Oddly enough, this has actually happened in the literal sense. The audio track from the final showdown of Alien is not actually Sigourney Weaver's voice; it's lifted from a porno. If you watch (and listen) carefully, it's pretty obvious that Ripley's lips aren't moving, even though there's a woman gasping and moaning on the soundtrack. I saw this movie once on the big screen after reading about this little piece of trivia on IMDb and got to see this scene in all of its....erm....glory. Sadly, I think it ruined the film for me forever, but it certainly provides an excellent example of the similarities between horror films and pornos.
Horror films are 'acceptable' in mainstream society. Pornos are not, although this standard has been weakening gradually over the past few decades. Both attempt to tap into humans' 'primal instincts,' and, naturally make money off of people's collective needs. Since the appearance of the slasher film, sex has played an extremely important role in horror movies. Sex is rather important to pornographic films as well, naturally. Slashers play more into violence and that sort of power struggle while pornos just take sex to the extreme. Sex and violence are leaders among the most controversial and censored subjects in any form of media, which is why they are also used to attract people. There is the appeal of the forbidden, that which is unacceptable to polite society, and underneath it all, many are intrigued by it.
Then again, there's always the legend of the snuff film, a combination of the two. | 
