There are some very poor arguments dotted around in this thread. This issue boils down to simple visceral disgust. If it disgusts you, you state that it is wrong, morally. Reasons provided for the most part seem like clutching at straws in an attempt to legitimise and best convince others of your disgusted position, even though none of these reasons actually led to your initial disgust. Order of nature? Risk of infection? Procreative impossibility? All irrelevant.
The key is to ask what we even mean when we say morality? One could say that immoral things are those things that disgust us, but clearly that is not strong enough on its own, as someone could be disgusted by homosexuality, but that doesnt make homosexuality wrong. Their disgust is real and their right to feel, but thats different from then claiming the object is inherently immoral.
To those that claim sexual contact with non-human animals is immoral, i would like your view on the alien hypothetical. As its logically possible for a race of super attractive aliens to arrive on Earth, would it be immoral to engage in sexual activities with them for pleasure simply because they are 'non-human'? Is that really what’s at the heart of the morality, a simple arbitrary difference in classification or genetics? I dont think so. This example is designed to cut through feelings of disgust, and get you to ask yourself what is truly wrong about contact with non-human creatures?
We are hardwired to feel disgusted by bestiality, i mean it makes evolutionary sense. I of course tend to feel disgusted also. But the immorality is not defined simply by the human-nonhuman distinction, it’s based on an awareness and respect for the mental lives of other sentient creatures like ourselves, and is immoral when one shows no regard for this.
Real arguments for the moral quality, in my opinion must be centred on the themes of abuse, assault and coercion. Its the conscious and illegitimate taking advantage of or harming of another creature, in a hypocritical contrast to how you live your own life thats wrong. Constraining the freedoms of others to gratify yourself is to exercise the very freedom you deny them, making you illegitimate and morally wrong in your action.