Premise 4, which, for the overwhelming majority, trumps *ANYTHING* you can possibly put forward: You're a sick pervert to even be THINKING about having sex an animal, and you should be beaten, jailed, killed, or beaten then jailed then killed, and the animal you've "contaminated" by having sex with it needs to be killed as the satan-spawn it must certainly be now that you've corrupted it.
You're forgetting a crucial factor here, dude. I'll grant, your intent is good, but there's a reality to be faced: In general, people don't approach this topic with anything that even smells a little like logic - it's almost pure "gut feeling", which can't be touched by any amount of rational consideration. It nullifies *ANY* possible justification you might offer, any logical thought, any rational consideration, anything other than the knee-jerk "You're a sick fuck! DIE DIE DIE DIE DIE!!!!" response.
There are a few exceptions, no doubt - There are exceptions to EVERYTHING. But I guarantee that if you were to walk up to a thousand total strangers and put forth the idea that sex with an animal is acceptable, what you're going to come away with will be the verbal equivalent (if not the physical actuality) of 995 punches to the face and/or kicks to your ass. Maybe even a bonus bullet-hole or three to help you on your way. That's what you're up against in the real world. The happy-happy-joy-joy little oasis of similar-thinking folks here at Zooville doesn't exist in the real world, and you'd be wise to figure that out real fast, before somebody actually DOES put a beatdown on you for being a disgusting piece of shit that thinks there's even a possibility that fucking Fido is the slightest bit acceptable.