The Reason Washington State doesn't have a law against sex with Animals is because it's not rampant like states that do have a law. It takes a lot of people doing something that people don't like for them to create a law banning it.
I'm sure it would go something like this
"Hey Bill, I just fucked a Sheep"
"Wow that sick man, you shouldn't do that"
"Hey Bill, I just fucked a dog"
"Jesus your a sick man"
"Hey Bill, I just blew a horse"
"God Damn will you people stop, somebodies got to make a law banning this shit"
Then the politicians start arguing the merits of a law banning animal sex,
The Anti-animal fuckers will create a moral outrage
"These fuckers are fucking animals and is disgusting, just think your Saint Bernard could be getting ready to take a shit and one of these low lifes will sneak up behind him and BAM, your dog will never be the same, how are you supposed to walk you animal proudly down the street knowing your neighbour had his way with him, the taughts of Whore...Well I tell you it's just not right"
The Pro-Animal Fuckers will try to bribe there local politicians with their finest pigs and chickens hoping ot sway the vote.
Final vote will come in 51-49 and the world is safe from the animal fuckers.