"It was just the most exciting, yummy, delicious experience of my youth," Sam Rader says. "I haven't been able to get it off my mind." The 31-year-old singer-songwriter and psychotherapist is swooning over her teenage years at an all-girls school, but don't make vulgar assumptions: She's talking about the chastest of pastimes. "Singing in harmony with people is a really transcendent experience," she gushes, "and I've been craving it ever since then."

Earlier this year, those yearnings led Rader to found the Silver Lake Chorus, a group of about 25 bright-eyed post-collegiates in Los Angeles that specializes in churchy, smiley, Hildegard von Bingen-meets-the Polyphonic Spree treatments of alternative-rock songs by the gelded likes of Beck, Radiohead, Phoenix, and Ben Lee. On the surface, the enterprise might seem absurd (hipsters go Glee!) but it represents the apex of a current pop-cultural trendlet: A cappella rock remakes are suddenly cropping up everywhere, from the opening set piece at August's Emmy Awards ceremony—where, along with stars from Glee, 30 Rock, and Mad Men, Jimmy Fallon turned Bruce Springsteen's "Born to Run" into a Broadway spectacle—to new acts like Chicago's Blue Ribbon Glee Club, which delivers Glee-ful YouTube renditions of Fugazi's "Waiting Room" and the Dead Kennedys' "California Über Alles," to the recent trailer for The Social Network, which featured Radiohead's "Creep" as reinterpreted by an obscure Belgian women's choir.

The phenomenon isn't limited to choral groups either. Take a look around and you'll see that alternative rock itself has suddenly gone all Mormon Tabernacle Choir on us. "Now our lives are changing fast/ Hope that something pure can last," Win Butler of Arcade Fire muses, wounded-childishly, on the Montreal collective's soaring anthem of the moment, "We Used to Wait." Purity has become, for Arcade Fire and peers with cheerful, cuddly names like Bright Eyes, Fleet Foxes, and Grizzly Bear, what broken glass was for Iggy Pop or nubile groupies were for Mick Jagger. Many of the most acclaimed and ascendant indie bands of this Green Day-goes-Broadway era write songs suffused with a prelapsarian sense of wonder—a tone that represents a sharp break from the world-weary, cerebral, acerbic vibe of the eighties and nineties alterna-heroes who paved the way. You can hear longing in the lyrics, yes, but it's a longing for a sort of idealized childhood, a "yummy" place of purity and hope, a Where the Wild Things Are-style Imagination Island where the healing can finally begin. What you don't hear is lust. Indie rock has never been the sexiest of genres, but lately it's become so nice and groin-averse that the guys in these bands can come across like sweetly militant eunuchs. The retreat into a quasi-juvenile cocoon makes sense—bombarded by bad news about the economy and borne back ceaselessly into the past by Facebook, your average twentysomething can be forgiven for clutching a security blanket. If rock and roll used to be about losing your innocence, now it's fixated on getting it back.

Which might, admittedly, be a smart marketing decision. Sex already saturates the music of pop and hip-hop contemporaries like Lady Gaga and Lil Wayne, and the swaggering loverman went out of style around the time that Rod Stewart first squeezed his package into leopard-print Lycra. "For the past 30 years or more, sex hasn't been such a taboo subject; after all, it's everywhere," says music writer Michael Azerrad, the author of rock histories like Our Band Could Be Your Life. "Being overtly sexual has simply lost its rebellious value."

Then again, purity can get kind of tedious, especially when it comes to rock and roll. "There's something Pollyannaish about it that's a turnoff," music journalist Marisa Meltzer says. "I much prefer anger and danger and sex in my music." You'll get none of that from the Silver Lake Chorus, as Dan Crane, a former SLC member who was kicked out earlier this year, knows firsthand. Crane, a solidly adult 39, says the sacking came as a relief. "To me the experience was like being in the drama club in high school and doing these massage-circle warm-ups, and these little facial exercises, and shake-out-your-leg-and-squeeze-your-butt-together stuff." Purity is wonderful in theory, but sometimes it can't hold up to the messy glare of reality. "There were so many times," says Crane, "when I thought, 'I can't believe I'm standing in a room with adults doing this.' "

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