Last week an obscure 2006 book on sexual ethics by a nun, a retired Yale Divinity School theologian, rocketed to number 13 on Amazon's bestseller list.

Its dazzling success came courtesy of the Vatican, which sparked readers' interest with its stern warning that Sister Margaret Farley's "Just Love" contradicted Catholic teaching. Using justice as a framework for sexual ethics, Farley had written that masturbation, gay marriage, and divorce could be seen as morally acceptable.

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