“Sex Tape” is not quite the train wreck its TV ads make it out to be. Which turns out to be the good news as far as this last and least of the big R-rated comedies of summer goes.

Cameron Diaz and Jason Segel slimmed down and earned the most flattering lighting manageable for the nudity, which is plentiful. They’re game enough. But their chemistry is strictly professional, and there’s just not a lot of fun in watching two actors go through the motions as boring suburbanites who dare to cut loose and try a no-holds-barred/every-position-tried sex tape, and then freak out when that video gets “out there.”

Diaz is Anna, who narrates an opening that recounts “the first time” her husband saw her naked. Diaz and Segel, as her husband Jay, make a spirited stab at playing their randy collegiate selves — making the beast with two backs in dorm rooms, cars and in between the library stacks.

A pregnancy and marriage follows, and Annie asks the only important question left to ask.

“When’s the LAST time your husband saw you naked?”

She’s narrating her blog (Who’s Yo Momma?) and lamenting the way kids, work and routine kill the romance and the sexual heat in a relationship.

Her blog is popular enough to merit attention from a big toy maker, which would like her to tame it and make her the face of their ideal mommy customer. Rob Lowe is the toothy, “I’m VERY excited” corporate kingpin courting her.

But the couple’s solution to their marriage doldrums could destroy that. Running through “every position” in “The Joy of Sex” on video for their new iPad may have been fun. What happens to the video that Jay “forgets to erase” is what sends them out, in the dark of night, to retrieve iPads they’ve given away to friends, family and their postman.

Yeah, I know, who DOES that?

The best moments in the middle acts involve Segel playing a deadly cat-and-mouse game with a German shepherd belonging to Lowe’s uber-rich toy mogul (He has Annie’s iPad).

But those moments are scattered as they team up with unfunny friends (Rob Corddry, usually better than this, and Ellie Klemper) and an obnoxious kid. Where a Judd Apatow or Nicholas Stoller or Seth Rogen would pile outrageous joke upon joke, Kasdan and crew dawdle from one half-hearted gag to the next. At a slogging 95 minutes, “Sex Tape” has no pacing and few rewards for enduring its dull stretches in between the better bits. Segel amps up the mania, here and there. But Diaz never unleashes that “Bad Teacher” fury.

“My gynecologist is going to be SO disappointed!”

Still, two troubling social ills are mocked in the film. Yes, our world’s over-connected, something demonstrated every time an iPad/Macbook or iPhone runs an app that uploads a video, or turns up creepy toy boss Rob Lowe’s fireplace, cranks up his favorite band (Slayer) or sets the mood lighting for when he asks if Annie is up for a little cocaine.

“You wanna bump?”

And yes, product placement has run amok in the movies. Seriously, did Apple underwrite this? Because the message isn’t going to help their bottom line. That message — “Apple will be our undoing.”

On paper it all should have worked. Diaz’s “Bad Teacher” director Jake Kadan behind the camera, Segel and his “Forgetting Sarah Marshall” pal Nicholas Stoller joking up Kate “The Back-Up Plan” Angelo’s script. Something goes terribly wrong long before Jack Black turns up as the foul-mouthed porn purveying voice of reason and marital harmony in the third act.

As Black’s character reminds up, no “Sex Tape” ever did anybody any good. It just spices up what’s already stale — a marriage, or a movie, especially one that Jay, as he’s making it, reviews himself.

“I’m not sure the story’s that important.”

MPAA Rating: R for strong sexual content, nudity, language and some drug use

Cast: Cameron Diaz, Jason Segel, Rob Lowe, Ellie Klemper, Rob Corddry

Credits: Directed by Jake Kasdan, written by Kate Angelo, JAson Segel and Nicholas Stoller. A Sony/Columbia release.

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