Bel Powley as Minnie Goetze in “The Diary of a Teenage Girl.” (Sam Emerson/Sony Pictures Classics)

Good luck seeing “The Diary of a Teenage Girl” — if you’re a teenager.

Since garnering praise at this year’s Sundance Film Festival, where it was nominated for a grand jury prize, filmmaker Marielle Heller’s adaptation of Phoebe Gloeckner’s 2002 semi-autobiographical graphic novel has generated steady controversy for its nonjudgmental portrayal of a 15-year-old girl who willingly — if naively — enters into an affair with her mother’s 35-year-old boyfriend. Kristen Wiig and Alexander Skarsgard play the grownups; 23-year-old British actress Bel Powley, making her American film debut, portrays the titular heroine, Minnie Goetze.

Although “Diary” has received an R rating from the Motion Picture Association of America, thereby opening it up to 17-year-olds — and, potentially, younger children with open-minded guardians — in England the film was slapped with the most restrictive rating, the equivalent of an NC-17, for what the British Board of Film Classification called the movie’s “numerous and sustained” sex scenes and references.

That’s more than just an “incredible disappointment” to Powley, who says she thinks younger teens need to see the film. In a recent phone interview, the actress called the BBFC rating a “national embarrassment.”

“It just goes to show that the reason that this movie hasn’t happened before is that society is scared of teenage girls,” says Powley, citing what she calls the double standard to which we hold teenage boys, whose lust is snickered at, and girls, whose libidos aren’t even acknowledged.

Marielle Heller, director of “The Diary of a Teenage Girl," on her own teen years: “ I remember feeling that I might actually die from a broken heart.” (Rebecca Cabage/Rebecca Cabage/Invision/Associated Press)

“We want to think of women as unsexualized beings when they’re children,” Powley says, “but then we ignore everything that happens in the middle, until they become women and they’re responsible and they get married and they use their sex to have babies.”

Heller acknowledges that she was pleasantly surprised by the MPAA’s response. “Working with them was actually a really positive process,” she says. “I was able to explain my position and why the film needed to show what it showed, and why I thought it would be hypocritical for the movie to get an NC-17. One of the things we talked about is why we’re more comfortable seeing a woman get violently raped [on-screen] than to see a woman having an orgasm.”

Though Powley’s character has several nude scenes, and takes a precocious pleasure in sex, the film avoids explicit images of genitalia, except for cartoonishly animated versions of those body parts. Heller says she knew she could get away with it “because the film ‘Superbad’ is full of drawn penises.” What’s more, Minnie is an aspiring comic artist, idolizing such transgressive underground cartoonists as R. Crumb and his wife, Aline Kominsky. “Besides,” Heller says, “I just think drawings of the penis are funny.”

Heller, an actress best known for her stage roles and a handful of small movie parts (“MacGruber”), originally wrote the part of Minnie for herself and starred in an off-Broadway adaptation of Gloeckner’s book in 2010. When she decided to turn it into a movie, the 35-year-old first-time director knew she would have to find a younger actress for the part. Ideally, it would be one who was as passionate about the project as she was.

According to Powley — who sounds remarkably like a 15-year-old when she says she thought she would “die” if she didn’t get the part — her homemade audition video was shot in her bedroom and included a personal appeal to the filmmaker. “I said, ‘Hi, Marielle Heller, I’m Bel Powley, and I’m in London. I can’t be in Hollywood to meet you in person.’ And then I told her some quite personal things, like why I related to this movie. I just felt that she needed to know that.”

Although Powley won’t disclose the precise details of that confession, she says it generally had to do with how deeply she related to the character, in her awkward heart, if not in Minnie’s specific circumstances.

“I was never this quippy, quick-thinking character that we see in so many coming-of-age movies, where nothing really fazes them and they just breeze through everything,” Powley says. “When I was a teenager, it felt like the world was going to implode if something went wrong.”

Heller echoes those extreme sentiments. “This is not my story, but it felt like it was about me,” she says. “My own teenage-hood was, for whatever reason, one of the most painful periods of my life, for all of the reasons that teenage-hood is painful, for both boys and girls. At that stage, everything feels like life or death. I remember feeling that I might actually die from a broken heart.”

According to Heller, there are several implicit morals to the movie: Love and sex are not the same thing; desire can be as powerful as being desired, if not more so; self-respect is better than a boyfriend. But both she and Powley resist called “Diary” a message film.

“We were never really intending, while making this movie, to do anything either revolutionary or radical,” Heller says, although several critics have used those words to describe the film. If there’s anything groundbreaking about “Diary,” the director suggests, it’s less a function of the big taboos that this story breaks — e.g., portraying a pedophile as sympathetic — than it is about our own expectations of Hollywood.

“It’s amazing how low our standards are when it comes to women’s stories,” she says. “Even when it comes to having a real-looking female body on-screen, that is almost considered revolutionary.”

Notwithstanding a title that makes “Diary” sound like a Young Adult classic in the making, this is a movie for grown-ups, and both Heller and Powley know that. Still, they hold out a sliver of hope that a few smart girls will see through the disturbing story line and take its larger points to heart.

“I’m just hoping that girls will see this movie and it will allow them to be more comfortable with their sexuality,” Powley says. “We watch so many movies about boys losing their virginity, boys having sex, crazy stuff like boys having sex with apple pies. Boys grow up knowing that all of those crazy sexual thoughts are normal. With women, it’s a taboo subject.”

The Diary of a Teenage Girl (R, 102 minutes). At area theaters.