One of the more interesting facts in Esther Perel’s new book, State of Affairs: Rethinking Infidelity, comes near the beginning. Since 1990, notes the psychoanalyst and writer, the rate of married women who report they’ve been unfaithful has increased by 40 percent, while the rate among men has remained the same. More women than ever are cheating, she tells us, or are willing to admit that they are cheating — and while Perel spends much of her book examining the psychological meaning, motivation, and impact of these affairs, she offers little insight into the significance of the rise itself. So...