I understand that some people are attracted to animals (and children, and inanimate objects and probably many other things I can't and don't want to imagine).
But largely speaking, animals don't want to have sex with people. They only rarely go into estrus and when they do, it is others of their species that they actually appear to want.
Animals can of course be trained to do almost anything. But that's hardly consent.
There are laws preventing people from having sex with the mentally retarded - specifically those under the I.Q. of 80 - under the assumption that they are incapable of giving consent. Not sure if non compos mentos is the correct term in this case.
There are no donkeys with anywhere near that kind of I.Q.