[NSFW] Why is this act illegal?

The question is: why is bestiality illegal?

Please read the details before forming lynch mob.

We domesticate animals which involves training them, using them on farms, for protection, as sled dogs and many more uses. Secondly, they are slaughtered for food, kept on farms and force fed, their young are sometimes slaughtered for food and much much more.

The question of their right to not be eaten or trained for any activity doesn’t arise then. So out of all these things sex is the most natural activity. Why then is there such a case against it and why is it illegal? Using protection you can prevent disease too.

Also please do not assume from me asking this question that I am into bestiality. The question came to mind after I was watching a documentary.