Once in awhile someone will ask me "What, if anything, is verboten in today's permissive, literate erotica?" The answer is that pretty much anything is fair game, but there are what are called the four deadly sins: four subjects that a lot of publishers and editors won't (or can't) touch. These by no means are set in stone, but they definitely limit where you can send a story that uses any of them. So here, in a special series, are theses sins, and what – if anything – a writer can do with them.
A story that features – positively or negatively – anything to do with sex with animals is tough if not impossible to sell, though some people have accomplished it. However, there are some odd angles to the bestiality that a lot of people haven't considered – both positive and negative.
On the negative side, I know a friend who had an erotic science fiction story soundly slammed by one editor because it featured sex with something non-human, technically bestiality – despite the fact that there is a long tradition of erotic science fiction, most recently culminating in the wonderful writing and publishing of Cecilia Tan and her Circlet Press (both very highly recommended). Erotic fantasy stories, too, sometimes get the "we don't want bestiality" rejection, though myth and legend are packed with sexy demons, mermaids, ghosts, etc. This doesn't even get into the more classical sexy beasts such as Leda and her famous swan or Zeus and other randy gods and demi-gods in their various animal forms.
Alas, "someone else did it" doesn't carry any weight with an editor and publisher, especially one that might be justifiably nervous about government prosecution or distributor rejection. Erotica, once again, gets – bad joke number three – the shaft: because erotica is up-front about the nature of its writing, alarm bells go off, unlike if you were writing something scholarly or even pop-culture. Market something as erotic and the double standards start popping up all over the place.
On a positive note – as the already mentioned, Cecilia Tan has proved – sex with aliens and mythological creatures has always been popular. Anthropomorphizing an animal, adding intellect or obvious will to a creature is a very safe way of touching on, or even embracing, the allure of sex with the unusual. The furry subculture is a close example of this, though they are very clear that this is not bestiality. It's just a way of eroticizing the exotic, mixing human sexuality with animal features. As long as the critters being embraced are not "real" animals and can give consent, then protests and issues usually fall away. Fantasy, after all, is one thing, and there's nothing more fantastic that dating a being from Tau Ceti V or something that looks like a raccoon crossed with Miss November, 1979.
There's another feature of bestiality that can be explored but only until recently has been: the idea of role-playing. In this take, a person will behave like an animal, usually a dog and usually submissive. In these S/M games, the "dog" (notice that they are never cats) is led around on a leash, communicates in barks or whines, drinks and eats from a bowl, and is generally treated – much to his pleasure, or as punishment – like a pooch: one-way it's a unique power game, read it another and it's bestiality.
One thing worth mentioning, because some people have brought this up in regards to all of the sins, is the dream out. What I mean by that is simple: say you really, really want to, say, write about doing some member of another phylum. That's cool, but your chances of seeing it in print, or even on a Web site, are about slim to none. SF doesn't turn your crank so you say: "Got it! It's a dream!" Well, I got news for you: a story that's slipped under the door with that framing device, as a way of getting about the idea of a real bestiality story apparent, especially when it opens with "I went to bed" and ends with "Then I woke up" is a pretty damned obvious excuse to write an un-sellable bestiality.
In short, like with a lot of these erotic "sins" whether or not a story comes across as being thoughtful or just exploitive and shallow depends a lot on how much you, as the writer, has put into the concept: something done cheap and easy will read just that way, versus the outcome if you invest time, thought, and – best of all – originality. Good work really does win out, and even can wash away some of the more outré' erotic "sins."