that's not how morality (or rules) works at all. morals are formed from an ethical system. deontologists have the categorical imperative, utilitarianism and hedonism have the maximization of pleasure, divine command theorists have the command of the deity, virtue ethicists have moderation between competing extremes. if any of them prohibit sex with animals, it's probably only divine command theory and maybe the categorical imperative. I guess the big "eww" factor could put off the virtue ethicists, too (bestiality isn't very aesthetic).

rules and laws are meant to keep social order. where they prohibit thing like killing or some other ethically bad thing, it is only a coincidence.