[shameus_o'reaaly]

It's one of those things that disgusts the majority so much they never need to think about WHY it should be illegal.
This kind of law is a positive step because it sets a precedent and clearly defines what the law regards as morally repugnant and aesthetically repugnant. Disgust is an important feeling-and more people should heed it-but it's not a basis for legislation by itself.
Screwing animals or corpses is repellent aesthetically, and while it's not in the same league as screwing children, it's an area of behaviour that needs to be "fenced off" even if in a criminal sense it's a grey area.
And "victimless" it may be as an act, but the consequences may cause harm beyond the actual participants. HIV is believed (though not known) to have mutated from an immunodeficiency virus suffered by monkeys, who transmitted it to humans by biting them. Flu supposedly jumped to people from pigs, leprosy from buffaloes-how much easier would transmission and mutation of a disease be if someone was screwing livestock instead of milking them or pulling a plough?