As you said, you haven't read the whole thread. But if animals copulate purely by instinct, then whether it's with another animal or a human it is rape by your description. Many vegetarians would consider eating meat to constitute murder. So, if it's rape, then meat is murder! It is very difficult to ascribe legality (or illegality) to any instinctual behavior, whether with humans or other animals. Under Sharia, what we would describe as rape is immoral fornication by the female if she can't produce 4 male witnesses in her defense. Who draws what line...?