Actually, it is rougher and randier than those movies, earning its R rating with a lot of naughty talk and a dose of semi-cynical sexual candor. The first scenes flail in the direction of various coarse comic subgenres, as we see an icky, awkward summer camp encounter, followed by a frat-house blowout in which we catch sight of Greta Gerwig drunk and wearing shorts with the word “whore” emblazoned across the back. Then there is a funeral, some family awkwardness and a drunken evening that leads to morning-after sex in the wake of a one-night stand that did not happen. It’s not really as confusing as it all sounds, but it is hard to escape the feeling that Mr. Reitman shot six or seven movies and then went into the editing room blindfolded to splice them all together.

The one that predominates is basically “Love & Other Drugs” without the disease. Like Anne Hathaway’s character in that movie, Ms. Portman’s is the one who insists on keeping things casual, physical and commitment-free. The arrangement that Adam and Emma agree upon is that they will have as much sex as they want — and Mr. Reitman’s way with montage suggests that it’s a lot — without the emotional entanglements that everyone watching knows are inevitable and that they both want. Emma’s resistance seems particularly unmotivated. She obviously likes Adam but refuses to fall for him, as if adhering to a deeply held conviction or trying to win a bet.

The pleasures of “No Strings Attached” are to be found in the brisk, easy humor of some of Ms. Meriwether’s dialogue and in the talented people scattered around Ms. Portman and Mr. Kutcher like fresh herbs strewn on a serving of overcooked fish. Kevin Kline, who played a goatish older literary gent in “Definitely, Maybe,” does something similarly mischievous and amusing as Adam’s father, a louche former sitcom star who takes up with his son’s ex-girlfriend (Ophelia Lovibond).

Mr. Kline has some room to work, but too many other actors are crowded into too few scenes. Chris Bridges (a k a Ludacris), as the owner of Adam’s favorite bar, could surely do more, and so could Olivia Thirlby as Emma’s radiantly monogamous younger sister. Lake Bell, as a smitten co-worker of Adam’s, is funny even as her character is set up from the start to be hurt and humiliated, a fate the film tries unconvincingly to soften.

Adam and Emma have the requisite nutty friends, though Adam’s best pal, Eli (Jake Johnson), is much blander than the crude dude you would expect to find in his position. The film’s great squandered opportunity — and also the source of some of its best comic moments — is that Ms. Gerwig and Mindy Kaling in effect share the role of Emma’s zany sidekick.