When Laura Sessions Stepp warned of the potentially damaging effects of "hooking up" in a new book, some people scoffed -- particularly those who believe they were unscathed by their own unfettered years of casual sex. Others, though, were incensed. In "Unhooked: How Young Women Pursue Sex, Delay Love and Lose at Both" (Riverhead), Ms. Sessions Stepp, a Washington Post reporter, writes about how smart, ambitious young women do emotional damage to themselves by getting physical -- making out to having sex -- with men they are not dating or may have met for the first time. In the book,...