The opposition to that law is simply due to the threat of other laws that prohibit non-normative sexual activity losing force.
Back in the early 1970s, a student at the university I attended was arrested after being caught having sex with a pig on the school's agricultural campus. It was on the front page of the local paper, complete with a photo of the young man. Outrage was general. The District Attorney in that county, however, declined to prosecute, saying that "some psychological help will have a better affect." The student promptly left town and no more was heard about this rare incident. No word on the condition of the pig was ever written.
Such things are extremely rare, and generally have to do with other problems in the people who do such acts. Aside from the general offensiveness of bestiality to almost all people, I cannot see prosecuting such offenders as a good answer. It's just a very sad, sad situation all around when it occurs.