Well, we're almost done here. BEHOLD:
Exodus 20:14, "Thou shalt not commit adultery," the Sixth Commandment.
Matthew 5:27-28, "You have learnt how it was said: You must not commit adultery. But I say this to you: If a man looks at a woman lustfully, he has already committed adultery with her in his heart," which reaffirms the previous passage.
In addition to laying eyes lustfully on the opposite sex, entertaining sexual fantasies in one's mind with animals is just as immoral, if not, worse. Man was created to rule over the animals, and each species was to multiply with itself. You already understand it...humans with humans, animals with animals, no mixing or cross-breading, just keep the natural law NATURAL.