We keep hearing that young men are failing to adapt to contemporary life. Their financial prospects are impaired—earnings for 25- to 34-year-old men have fallen by 20 percent since 1971. Their college enrollment numbers trail women's: Only 43 percent of American undergraduates today are men. Last year, women made up the majority of the work force for the first time. And yet there is one area in which men are very much in charge: premarital heterosexual relationships. [snip] What many young men wish for—access to sex without too many complications or commitments—carries the day. If women were more fully in...