According to a couple of studies, around 15% of women at some point in their lives fantasize about sex with an animal, most often a dog but also donkeys & horses figure in such fantasies a significant percentage of the time. So it's not a mainstream fantasy by any means, but it's not one-in-a-thousand either. As for morality, given the fact that human history is an endless litany of people inflicting horrors on each other, I think we have more important considerations than worrying about peripheral sexual predilections.