It's not the facts but the feelings that are important in telling young people about sex, says Kaplan, but this is largely a ""humanistic"" (a favorite word here) presentation of the facts--from cell division and animal reproduction to human sex organs and how they work. . . on through the physiology of the sexual response and the choice between pads and tampons (or briefly, diaphragm and pill). On the feeling side are such problems as performance-anxiety and orgasm watching, both alleviated by fantasy. Kaplan credits Freud, Kinsey, and Masters and Johnson for their separate breaks through taboos against sex study. Her own viewpoint is unabashedly straight--to Kaplan, sex means intercourse, and homosexuality is a disturbance treated fleetingly here with such other variant forms as fetishism and exhibitionism. This differs from most sex advice for the young, however, in being truly nonjudgmental about premarital and teenage sex--Kaplan's ""no one can make this decision for you"" is not a preliminary to card-stacking cautions. And the numerous informational drawings manage to be clinically accurate without being cold or ugly.