I don't pay a lot of attention to the ratings on films. Whether a film is rated G, PG, PG-13, R or NC-17 has little affect on me. I am in my 40's and I don't have any children. But sometimes I have to scratch my head as to why a film is rated what it is. If you are at all curious about this, I suggest you check out the film This Film Is Not Yet Rated which you can stream via Netflix or IFC's web site.
This film is about the MPAA (Movie Picture Association of America) and spends a lot of time on the decision process involved in rating a film NC-17 which is the rating that they give films that children are not allowed to see in the theater. They are not quite as explicit to warrant an X rating (aka pornography) and generally have more of a plot than an X rated film. NC-17 means no children under 17 admitted under any circumstances. There are not a lot of hard and fast rules about ratings. They are decided by a group of anonymous parents who watch the film and decide who should be able to watch it. The difference between an NC-17 rating and an R rating can mean millions of dollars because the NC-17 rating can limit the distribution of the film. Sometimes the offending scene is a few second long while other times, the entire film would need to be re-edited.
I find some of the decisions made by the MPAA to be very odd. They seem very concerned about sexual content and have little concern over violence. Boys Don't Cry, Blue Valentine and The Cooler, good films with relevant social commentary, were initially given an NC-17 rating due to some fairly innocuous sexually content. The Cooler received the rating due to the exposure of Maria Bello's pubic hair. Yet, the film Scary Movie is rated R and depicts a cheerleader being decapitated and her head thrown in a lost-and-found bin. If I were a parent, I'd be more concerned about gratuitous violence against woman (or violence in general) than a pubic hair.
This wouldn't be a problem with me if the MPAA didn't have a monopoly and since their raters are anonymous they are beyond scrutiny. If there were other pre-distribution rating agencies, people (parents in particular) and theater owners could choose the rating system that better suited their values and/or tastes. Until then, I don't take the ratings from the MPAA very seriously.