Drawing on the post-Kinsey explosion of research on sexuality--including his own--Weinrich (research psychiatry/Boston U.) here examines and attempts to explain virtually all variations in human sexual behavior. Weinrich postulates that there are two kinds of sexuality: limerence (an attraction to a whole person--his/her character, talents, status, etc); and lust. Women usually experience limerence first, lust later; for men the reverse is true. This theory, says Weinrich, helps explain why men are turned on by pornography using lesbians, why there are more bisexual women than bisexual men, why women are more tolerant than men of male homosexuality, etc. He also cites evidence that homosexuality and bisexuality (not uncommon throughout the animal kingdom) may have a genetic basis and may also have a positive role in the survival of the species. Various Indian and Eskimo tribes, for instance, have bedarches--men who have been raised as females. Because they understand both male and female points of view, they frequently serve as matchmakers and mediators for the tribe. The recent proliferation of homosexuality, says Weinrich, may be the result of an evolutionary process to diversify humankind (even at the expense of some reproduction) in order to supply people with the enormous diversity of skills required by our technological society. He also suggests answers to such other ""puzzles"" as: why heterosexual men can be turned on by pictures of male buttocks and little girls, why some people become sexual exhibitionists, why sado-masochism is associated with sex, and why there are far more masochists than sadists. Controversial, replete with enough informational tidbits to dine out on for months, and likely to stir up some media attention.