THE ETHICS OF ZOOPHILIA:
Though I have no personal interest in having sex with animals I've been asked about the topic of bestiality or zoophilia so I've decided to respond. There is quite a bit of literature on the ethics of zoophilia. See http://en.wikipedia.org/wiki/Zoophilia for a summary.
The view of many ethicists (e.g. Peter Singer) who have considered the issue, and a view to which I largely subscribe, is that sex with animals is ethical if no harm is done to the animal, that is it is not physically harmful and it is not particularly coercive or stressful, no more so than humane pet ownership would allow. I will note in passing however that much of what is often considered normal pet or food animal ownership is itself certainly often gravely harmful both physically and psychologically to the animals in question.
No doubt in many cases such as the most common variety, that of male dogs with women, it can be pleasurable to the animal as well as the human. Statistics are difficult to come by but there is quite a bit of anecdotal evidence that quite a few women take male dogs as lovers, and sometimes female as well for oral sex. One can find an enormous volume of videos and photos on this subject on USENET (a check of the bestiality group on 2/15/10 showed 6.9 million! current posts most apparently images and videos of women with dogs) and it is apparent that the fantasy of reduction of human females to pure animal sexuality has considerable appeal to both men and women. There are probably quite a few advantages to that from both the dog's and the woman's perspective, compared to taking a human male lover with all the messy complications that can entail!
As to the risk of disease I'm sure there must be some, but then again the risk of disease transmission from human to human sexual contact is considerable and might be even more worrisome.
There is certainly the danger of abuse and injury depending on species and circumstance however. Chickens or ducks are occasionally mentioned as sex objects in traditional songs and tales. They, as all animals certainly experience pain and stress and certainly can be traumatized by abuse. I would suspect that cloacal sex with a chicken or duck would likely cause stress, pain or injury. The cloacal dilation in egg laying very likely requires a natural relaxation process which would absent in forced sex by a man.
Likewise I would suspect that a man having vaginal sex with more or less any animal especially one smaller than himself is likely to cause harm or injury due to the non receptive sexual status of the female animal which unlike human females only comes into heat in season. If the female is in heat and receptive to such advances then possibly but I think that fairly unlikely.
There was a 12/3/08 story in the Daily Telegraph of a UK man getting three years for having sex with a horse. I broke into laughter as the horse was pictured with a black bar over its eyes presumably to protect its identity! Here are my comments to the discussion group the article was posted on: Did the horse file a complaint? Was the horse under the age of consent? Will the horse testify in court? Why does the horse have a black censorship bar over its eyes to protect its identity?
The article claimed that the act terrified the horse. Somehow I rather doubt that. If it had the horse would simply have kicked the guy's balls off. Anyway the horse was obviously asking for it since it wasn't wearing knickers. :-) All kidding aside, I suspect the horse was large enough no damage was done, certainly none physically and unlikely any psychologically. I'd say the man ran the much greater risk in this case.
In summary I'd recommend that the legal approach to zoophilia should be based on harm to the animal if there is any. That criminality should always be judged by its effect or lack thereof on the 'victim' is a general principle of justice I develop on the section on Justice and there is no reason that it should not apply to animals as well as humans with few exceptions.