If a movie shows a person's bare backside should that movie be rated R?

I just watched a movie and though the movie was rated PG13 I felt vary uncomfortable with seeing one of the male actors bare backside over and over again. I think this movie should have been rated R(a mild R but still a R). Do you feel the same way I do about nudity in movies? Should a movie that show a person's fully nude body(front or back) be rated R?