This might be disgusting to the majority of readers here, but it is NOT (inherently) wrong.
Yes, I understand that sex with animals is taboo and irks most people out, but that doesn't make it cruel, or should consider it a crime. A lot of people jump to the first problem that comes to mind as a rationalization why it's wrong. Here is where the arguments fall short and what the facts are.
The popular one is to compare animals to kids, probably because so many people see and treat their animal companions as kids. Huge problem with that, kids are not sexually mature, whereas all adults and grown animals ARE sexually mature. Likewise, animals hunt, have sex, reproduce, look after their young, and care for themselves in the wild, unlike children.
Intelligence and communication are also a popular argument to make, as if animals don't have any and therefor can not (willfully) participate. Good luck going up against an increasingly growing wealth of knowledge about the sophistication of animal communication and intelligence. So far our understanding of animal intelligence seems to be limited more by our ability to accurately measure animal intelligence rather then the limit of their cognitive abilities. Regardless, having less or a different kind of knowledge doesn't stop two individuals from enjoying sex. In addition, anyone with a close relationship to a companion animal generally has a good knowledge of how they communicate. Even the densest of people can understand when an animal is excited, scared, or *ahem* horny.
The last one I'll cover is one that comes out of the disgust factor. The way people describe it, you are almost certain to catch a parasite, disease or germs as soon as you come into sexual contact with an animal. The reality is, a zoophile and his or her animal partner is less at risk then a typical person. Germs are very species specific, so kissing fido is probably safer then kissing your wife/husband. Parasites in animals are easily prevented (or treatable). Again, most diseases are species specific, or aren't passed solely by sex. Finally, no risk of babies. Different species can have sex but aren't able to reproduce.
Bonus argument (or the one people wont admit to): Human exceptionalism. The frame of mind for this one is that we humans are the best species, most advance, most intelligent, so superior and have a god given right. So all other species (despite their ability to feel and have emotions just as we do) are beneath us. In fact, they are not communities or individuals with their own interests, but objects and resources to be exploited. Here in lies the problem, having a sexual partner implies that the two are equals.... and if that partner is a non-human animal, that contradicts our paradigm above.
*patiently waits for vitriolic replies*
