Take that, Norma Klein. In the permissive California atmosphere of the Edgemont Swim & Racquet Club, teenage sex is dirty, dirty, dirty, especially the brother-and-sister incest variety. So you can count on sufficiently tragic comeuppances ("". . . their life is shattered, their days ridden with torment. . ."") for eighteen-year-old near-nympho Peach--it's rumored that she ""walks around with a mattress strapped to her back to save time""--and horny, likably loony kid brother Kevin; left alone in the house one weekend, they groove together so well (Kevin, having read Peach's hot diary, is the blackmailing seducer) that they run off for a week in bed at the beach. A dum-dum scenario, counterpointed with racial tensions around the club, but Asher, so musically entertaining some years back in Piano Sport and Electric Cotillion, can cater country-club dances and dinners as convincingly as anybody. And the frantic couplings are arranged no more salaciously than they have to be to support an unstuffy but old-fashioned response to new fashions in sinning.