Even as moralists and activists continue to debate homosexuality, many species casually practice it. What humans share with so many other animals, it now appears, is freewheeling homosexuality. Nature's morality it seems, may be remarkably flexible, at least if the new book Biological Exuberance (St.Martin's Press) by linguist and cognitive scientist Bruce Bagenugli, is to be believed. According to Bagemihl, the animal kingdom is a more sexually complex place than most people know-one where couplings routinely take place not just between male-female pairs but also between male-male and female-female ones.