Nice advice for the frantic parents of post-pubescent products of the second generation of sexual revolution from Kappelman, a psychological and medical counselor to teenage children. The question, apparently, is no longer if a teenager should engage in sexual relations, it's when. And when, according to the author, is when the relationship and the child's level of maturity (not just his hormones) are ripe. That doesn't sound like much on the face of it, but Kappelman's tone is so reassuring, so rational, so. . . lulling, even, that this can probably ease the jitters for a considerable number of parents who may not know how to deal with discussions of contraception, premarital pregnancy, cohabitation in college, homosexuality, and the like when they stop being hypotheticals and turn into issues.