Has bestiality always been historically so taboo? Why? It doesn't result in pregnancy; did it result in diseases? Why did humans evolve to get so repulsed from touching genitals with other species?
Do other mammals lack this "taboo"? Sometimes closely related species will mate (like lions and tigers, or humans and neanderthals), but where else does this happen? It is probably instinctive.
(Don't count horny adolescents that will rub against anything.)