As Billy Crystal’s character tries to prove in Nora Ephron’s classic romantic comedy When Harry Met Sally, men and women can never be friends because, according to him, “the sex part always gets in the way.”

In No Strings Attached, the sex part is the least ofAshton Kutcher’s and Natalie Portman’s worries, as they seek to discover whether men and women can maintain a friendship and resist developing feelings for one another while having sex — and lots of it.

The answer to this question, as anyone who has seen a romantic comedy or has common sense would know, is no.

The premise of the film is simple enough: Boy meets girl at summer camp while the two are in junior high, then boy meets girl again in adulthood. When boy and girl use each other for sex, complications arise.

A doctor working all hours of the day, Emma (Portman) instigates this purely physical relationship. Independent, strong-willed and slightly intimidated by affection, she fits the prototype of the 21st century female lead.

Emma also establishes the ground rules: No staring longingly into each other’s eyes, no jealousy and absolutely no spooning under any circumstances.

But rules in the movie world are meant to be broken, and this is where Adam (Kutcher) comes in. He agrees to become a “sex friend,” but is vulnerable, and yearns for something more from Emma.

The role reversal is refreshing yet it feels idealized. The two leads act like over-dramatized versions of real people, without letting their roles develop any depth or complexity.

But it’s not entirely their fault. Every time the film approaches an area of substance, what feels like 20 superfluous supporting actors jump in with a punch line or two.

Although Emma’s comedic roommates (Mindy Kaling, Greta Gerwig and Guy Branum) are a delight to watch onscreen, the casting of rapper Chris “Ludacris” Bridges is so off-key and random that it begs the question of why he is needed in the first place.

No Strings Attached follows a formula that is tried and tested by every prior romantic comedy. DirectorIvan Reitman, of Ghostbusters fame, just barely bumps the movie to its R-rating.

Unfortunately, his attempt at the raunchy yet smart humor of Judd Apatow is more slapstick than witty.

Fresh off her critically acclaimed and haunting performance in Darren Aronofsky’s psychological thrillerBlack Swan, a role that snagged her a Golden Globe, Portman slides into the role of Emma like a bull in a china shop.

Clearly this genre is not for her, but she puts up a commendable first-time effort.

Kutcher, at least, has learned to embrace his inevitable fate of always being second best to a more dominant female lead. Here he is an accessory, but an affecting one nonetheless.

Clichéd, predictable and soon to be forgotten, No Strings Attached takes audiences on the same path that most romantic comedies tend to travel.