I have written an article on R-rated movies from an LDS perspective. When I wrote this article I was a student in the BYU Animation program.
If you are not LDS but have sometimes wondered why your Mormon friends are picky about what they watch, or have "black and white" rules about movie consumption, the article should answer your questions. It is, however, written to a Mormon audience. There is a division in Mormondom (if that's a word) on this subject, because some are pretty strict about it, but the official rules I don't think are quite so clear-cut, leaving the choice to the viewer's discretion.