Okay, as regards how natural or accepted bestiality is, it really depends on the place you're in. That link is pretty heavily NSFW especially at the end, so be very ware.
In summary, in Colombia, according to that video, it is common for most boys to have their first sexual experience with a donkey. I personally find that gross as hell, and would never do that, but to say it's not normal, or not accepted is dependent on perspective.
I suppose in the end it depends what you're having sex for and why it's important to you. For someone who considers sex to be an integral part of a relationship or marriage, this would be immoral. For someone who thinks sex is about gratification, it might be justified, but only if one negates or otherwise ignores the consent issue. An animal cannot consent to sex, it simply lacks the capacity to be informed. As such, the only way you might state an animal is okay to have sex with for gratification is where one believes that an animal is worth either less than a human to a sufficient degree that mere gratification is enough to override that value, or that animals have no inherent value and therefore are to be used as humans see fit.
I personally come down on the opinion that sex is a profound thing between humans, and to do that with animals is a denigration of what it is to be human. I don't think having sex with animals is okay, and I think it probably harms them more often than not.