Almost exactly a year ago, writer Katherine Dee, who blogs about internet culture and trend forecasting, predicted what she called a “coming wave of sex negativity.” Sex positivity, she suggested, had created new stigmas, including around discussing the harms of sex work and self-commodification. “People do not want to be atomized,” she wrote, adding, “Nobody wants this dystopia.” Not everything Ms. Dee foresaw — like a shift toward earlier childbearing among the upper-middle class — has come to pass, at least so far. But she nailed an emerging movement, one that now has a manifesto in “Rethinking Sex: A Provocation”...