Why do people make sex tapes, anyway? A souvenir? Future reference? Is it just one of those kitschy fetishes, like mirrors on the ceiling?

For Jay and Annie (Jason Segel, Cameron Diaz) in Jake Kasdan's mildly spicy comedy "Sex Tape," it's a way to reignite their erotic fire. The days of quickies in the college library are gone, replaced by the demands of marriage, work and children. "The last time was after we bought those towels at Bed, Bath & Beyond," Jay says wistfully. But with the kids safely at grandma's one night, Jay and Annie seize the moment. Armed with an iPad and "The Joy of Sex," they document a three-hour gymnastic marathon that leaves them exhausted, a little embarrassed and -- for the first time in a while -- extremely happy.

This first third of "Sex Tape" is funny, intimate and rather sweet, thanks largely to Diaz. Her Annie is a rare bird in the movies: a normal woman with a strong sexual appetite. (Kate Angelo, of "The Back-up Plan," wrote the script.) Annie is neither femme fatale nor tragic tramp, but a loving wife and mother with an adventurous side. In fact, the sex tape is entirely her idea. Segel's Jay is blandly likable, but Diaz -- with and without her clothes -- gives Annie a bouncy, zesty energy.

When the plot finally begins, it feels like an (ahem) anticlimax. Thanks to a syncing mistake that any Apple user will find convincing, the video is circulating around town on several iPads and perhaps even -- oh, God -- on the Internet.

Rob Corddry and Ellie Kemper play the couple's semi-helpful best friends, while Annie's seemingly wholesome boss, Hank Rosenbaum, is played by Rob Lowe, gamely satirizing his checkered past (though steering clear of his own famous sex tape from 1988).

Given its subject matter and R rating, "Sex Tape" isn't as wild or woolly as it could be. It's almost a cautionary tale whose moral is delivered, oddly, by a porn mogul (Jack Black). Making a kinky video might be a "quick fix," he says, "but it can't solve the bigger problems."

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PLOT An adventurous husband and wife discover that their private video is about to go viral.

RATING R (strong sexual content, nudity, language and some drug use)

CAST Jason Segel, Cameron Diaz, Rob Corddry, Rob Lowe


BOTTOM LINE Basically an R-rated sitcom episode, though the humor is surprisingly tame given the topic.



Here are Cameron Diaz's highest-grossing movies (adjusted for ticket-price inflation). Only time will tell whether "Sex Tape" makes the list.

1. Shrek 2 (2004) -- $565,565,400

2. Shrek (2001) -- $376,433,500

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3. Shrek the Third (2007) -- $373,379,500

4. There's Something About Mary (1998) -- $298,329,900

5. Shrek Forever After (2010) -- $239,037,100

6. The Mask (1994) -- $228,400,100

7. My Best Friend's Wedding (1997) -- $220,452,200

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8. Charlie's Angels (2000) -- $184,844,000

9. Vanilla Sky (2001) -- $139,850,900

10. Charlie's Angels: Full Throttle (2003) -- $133,102,400

Source: boxofficemojo.com