“Sex Tape” is a lump of half-cocked misunderstandings and nakedly affected zaniness.

There’s not much to this pratfalling exercise in futility that we haven’t seen before except for a flash of Cameron Diaz’s bare rump. And that is hardly an experience worth 100 minutes of your one and only life.

The film appears to have been written backward, title first, then a wet string of comic situations and one-liners, story last. Diaz and Jason Segel play Annie and Jay, college sweethearts in a monotonous marriage. She mommy-blogs, he is a music executive. Sensual ecstasy is a dim memory. To revive their relationship, she suggests starring in their own private porn video. Before you can say “viral,” their featurette migrates to a handful of matching iPads that Segel has given as gifts to friends, auto-synched to his ever-changing music collection. Aghast, the duo race to retrieve the devices before anyone has a chance to click Play.

At this point we are forced to draw several conclusions. First, Annie and Jay must have attended Duh University. Second, Diaz, who turns 42 in August, is simply not credible as a woman 10 years out of college. Third, director Jake Kasdan has zero feel for the cadence and rhythm of sex farce. The scenes that should be steamy or silly are merely squirmy.

The story proceeds as if it lost its driving directions. Annie’s blog is about to be bought by a wholesome baby-products company. Since Annie has given the firm’s boss another of Jay’s linked iPads, the pair must fast-talk their way into his house and steal it back. The film becomes a caper comedy that does not caper comically.

Rob Lowe’s appearance as Annie’s prospective boss adds a weird metatextual dimension. In 1988 Lowe was one of the first screen performers to be embarrassed by the release of a sex tape. He tried to spoof his bad-boy image at the next Oscars telecast by waltzing with an actress dressed as Walt Disney’s wholesome Snow White, a stunt considered a historic PR misfire. Here he plays a squeaky-clean corporate honcho with a few kinky secrets and a fetish for Disney characters. This makes no less sense than the rest of the film.

Diaz delivers a face-scrunching performance that would have the patrons of a dinner theater rattling their silverware in annoyance. Segel, whose comic chemistry with Amy Adams was enchanting in “The Muppet Movie,” seems adrift in these slackly staged proceedings, self-conscious and staring vacantly into the eyes of his leading lady. The dialogue (which he co-wrote) is a grab bag of balky phrases screaming to be polished into better jokes.

“Sex Tape” apes its frustrated protagonists, limply yearning for thrills it can’t achieve.