In the UK bestiality is illegal for obvious reasons such as animal abuse and also because the act itself is repulisve, I myself think it is. But I do not think this justifies it being illegal.
Animals are abused by humans in many different ways as well, such as chickens being battery farmed, living in cramped conditions serving an existence to merely be our food. We destroy their habitats around the world, kill intelligent animals such as dolphins for meat, use them in dangerous medical tests.
Animals are also abused in cosmetic testing, something which has no real benefit for society except to make girls think they are pretty and feel better. Think of the number of animals who have subjected to this kind of torture.
So with all the abuse that humans do to animals for our so called "benefit" then what is so wrong with a few individuals practising bestiality. After all our treatment of animals shows we regard them as lower life forms so whats so wrong about using a lower life form for our benefit in this situation?