Yes, Virginia, there is such a thing as a bisexual and Julius (Body Language) Fast has interviewed about a dozen men and women who champion this AC/DC lifestyle as the greatest thing since bottled beer -- for reasons which range from a candid ""I lost my fear of being gay when I became bisexual"" to a completely disingenuous ""I think it's a good way to keep my family together."" The men and women interviewed here all think they're genuinely bisexual but according to clinical psychologist Hal Wells who studied Fast's interviews, only about half of them swing both ways with equal enjoyment -- the others are hiding their actual identity. Fast suggests that true homosexuals invariably reveal some hostility to the opposite sex, while happy mix-and-matchers like Angela smile and say disarming things like ""I think my bisexuality just evolved out of my sexuality."" Ideally in this kind of life all sex roles, all competitiveness are eliminated and one searches only for a simpatico person. The trouble is that bisexual couples seem to have all the problems of straight people, only more so. What to tell the kids? Will the spouse accept it? Who does the shopping and the laundry? In an appended interview with Dr. Wardell Pomeroy, co-author with Kinsey of Sexuality of the Human Male, Pomeroy explains that sexual preferences range from zero to six on the Kinsey scale -- zero for totally hetero and six for totally gay and on top of that how people measure psychologically and how they measure when judged by their overt acts can diverge widely. . . . It's all fairly complicated trying to sort out who is and who isn't, and to what degree, and apart from a certain inevitable voyeuristic interest, you might just as well be grading tangelos and trying to decide whether they're latent oranges or tangerines.