From G to NC-17, one consistent principle in movie ratings is that female nudity usually equates to an R rating, while male nudity often leads to an NC-17 rating.

So why are women's and men's bodies not put on a level playing field?

It all started with "Bruno," Sacha Baron Cohen's R-rated, 2009 comedy. This movie featured full-frontal male nudity that prompted protests from parents and spurred the MPAA, which sets movie ratings, to add the term "male nudity" in 2010 to its list of descriptors for films that warrant an R or NC-17 rating. (An R rating means that no one under 17 may view the film without an adult; an NC-17 rating means that no one under 17 may view the film at all.) There is no comparable descriptor for "female nudity." Instead, when a woman appears naked in a movie, it is subsumed under the generic warning of "nudity."

Related: New Data on Female Occupations in Movies and TV is Troubling

Since the change, the descriptor "male nudity" has appeared in parental warnings for such films as “Jackass 3D,” “Eat Pray Love,” and “Grown Ups.”

Why is the MPAA more worried about male nudity?

The "male nudity" change sparked a backlash against the MPAA for employing a double standard when it comes to nudity. And that conveys the idea that female nudity alone is somehow more acceptable or is not sufficiently objectionable to warrant its own specific warning for audiences.

The MPAA, as you might expect, does not agree that film ratings and the modifier of "male nudity" are sexist. Instead, it says that the special note for male nudity is just an effort to offer specifics to parents of young children. Nor does the MPAA agree that male nudity always earns an NC-17 rating.

"We see 700 to 900 films a year," Joan Graves, Chairman of the Classification and Rating Administration at MPAA, told ATTN:. "In some cases, once we give a rating, the submitter has asked us to be more specific, for example, identifying male nudity. It is their request that we are more specific with what kind of nudity is in the film. The important thing is how parents perceive it and how involved the nudity is. If parents would perceive it as strictly adult content, than it could be NC-17. But male nudity is not an automatic NC-17 at all.”

But not everyone agrees. “Fifty Shades of Grey," for example, featured full-frontal nudity on the part of its lead actress, Dakota Johnson; the film carried an R rating. By contrast, “Shame” featured an image of a fully exposed Michael Fassbender; the movie carried an NC-17 rating.

This paradox is also present in the film "The Sessions," starring an often-nude Helen Hunt and a clothed male costar, John Hawkes. Ben Lewin, the writer and director of the film, told the Los Angeles Times that his "task was not to challenge the MPAA," adding that any male frontal nudity, especially in an aroused state, would guarantee an NC-17 rating. Because an NC-17 rating limits audiences and sales for the film, producers stay away from male nudity. From 2006 to 2010, 786 films carried a warning of "nudity," but only three warned of "male nudity."

Such ratings suggest that only mature viewers can handle male nudity, while younger viewers can be exposed to female nudity without a problem.

The result? We see a lot more female nudity than male nudity.

According to the New York Film Academy, in the top 500 critically ranked films from 2007-12, over 26 percent of female actresses appeared partially nude on screen versus less than 10 percent of men. Contrary to the large number of famous actresses who have appeared nude, such as Jennifer Connelly, Nicole Kidman, and Naomi Watts, fewer prominent actors have bared it all, as did Ben Affleck in “Gone Girl.” The sheer volume of actresses appearing nude onscreen has helped make female nudity the norm. Male nudity remains a novelty.

And there might be homophobia, too.

Related: One Group Had Literally Zero Representation in 2014's Top 100 Movies at the Box Office

The MPAA's ratings can not only be viewed as sexist, but possibly homophobic as well. Many films showcasing sex scenes of straight couples have gotten an R rating by the MPAA, including "No Strings Attached" and "Friends with Benefits." Yet, "Blue if the Warmest Color," a story that focused on a lesbian couple, received an NC-17 rating to the shock of many movie goers, many of whom believed that the same-sex relationship was the only reason for the tough rating. In response, Manhattan's IFC Center went so far as to admit teens into the showing, as a protest against the MPAA's rating.