‘Sex Tape’ fails where it tries to be funny

The cinematic equivalent of herpes, “Sex Tape” is an uncomfortable embarrassment to raunchy comedies everywhere. Fortunately no medication is required after being exposed to it: The effects are not permanent, only painful.

Here’s the weird thing: Two of the movie’s three writers (Jason Segel and Nicholas Stoller) have collaborated on a couple of good movies (“The Five-Year Engagement,” “The Muppets”). Segel also wrote the even funnier “Forgetting Sarah Marshall,” while Stoller directed. Where did things go so horribly wrong with “Sex Tape”? Did the infection stem from the inclusion of writer No. 3, Kate Angelo, responsible for the dreadful J-Lo movie “The Back Up Plan”?

Hard to say. “Sex Tape’s” basic premise is solid: Married couple Annie and Jay try to jumpstart their sex life by making a pornographic video that inadvertently ends up on the iPads of friends and family (the movie is basically Apple’s worst ad ever). Stars Segel and Cameron Diaz (whose last adult comedy, “The Other Woman,” was one of spring’s most unexpected surprises) have made reliably funny movies in the past. Even the supporting cast – which includes Rob Lowe as the outwardly clean-cut CEO who wants to hire Annie as a sort of spokesperson for his family-oriented company – is more than capable of producing laughs.

But everything about “Sex Tape” feels off: the comic timing, the pacing, the jokes, the fact that Diaz and Segel play their college-age selves. Even the editing is choppy, the explanations of how the video got synced to everyone else’s iPad clumsy and repetitive. The result is a movie that feels thrown together haphazardly. Director Jake Kasdan (“Bad Teacher,” “Walk Hard: The Dewey Cox Story”) seems to have assumed that the subject matter is enough to make the audience laugh, and he’s wrong. There are a few amusing sight gags involving Segel and a German shepherd (animal lovers may disagree), but almost every other joke is a retread. The movie even throws in the overused let’s-do-coke- and-get-all-crazy-and-talk-fast scene, a sure sign of desperation.

Segel and Diaz are game for anything – both of them have nude scenes, and Segel even manages a graceful if startling naked headstand – and at the film’s end, when the audience finally gets a glimpse of the sex tape in question, a few horrified laughs may erupt. But at that point, it’s too little and too late to get worked up over a movie that’s not even half as crude, rude and funny as it could have been.