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?
I don't think people being concerned about animal cruelty was the reason why it evolved as taboo in the past; people have been doing terrible things to animals with no guilt both in the past and today, so I suspect the reason why it's taboo is different.