Anti-speciesism

Let's be clear: bestiality or zoophilia is unequivocally the sexual abuse of an animal.  Piers Beirne, Professor of Criminology at the University of Southern Maine, summed it up quite clearly in 1997 when  he wrote that “for genuine consent to sexual relations to be present...both participants must be conscious, fully informed and positive in their desires. Bestiality is by nature sexual coercion because animals are incapable of genuinely saying ‘yes’ or ‘no’ to humans in forms we can readily understand.”

It is potentially harmful to the animal, depending on the size of the animal and role that they play. There may be physical damage and, in the case of male dogs, they may become dominant and pose a risk to other people. 

In addition a study by William M. Fleming et al "...compared the family characteristics, victimization histories, and number of perpetration offenses of juvenile offenders who admitted to having had sex with animals to juvenile offenders who did not. The study found that 96% of the juveniles who had engaged in sex with nonhuman animals also admitted to sex offenses against humans and reported more offenses against humans than other sex offenders their same age and race." http://booksandjournals.brillonline.com/content/journals/10.1163/156853002760030860 

Not all cultures consider bestiality to be a criminal offence and ban it directly. Some consider it to be just a form of abuse or cruelty, whereas others tolerate it or even allow it.