Sometimes I get the impression that U.S. judges and prosecutors have a greater freedom how to mount a case or to choose the punishment than their brothers in other ocuntries, but overall the rule is said to be: if something doesn't fit an act or a kind of action that's criminalized by the law, then you can't take it to court under penal law.
You could mount a civil lawsuit, but i don't see anyone making a lawsuit about bestiality - if the thing isn't criminal, then it would have to be a case about cruelty (to the animal or the human) or mental abuse. You could argue it's immoral of course, but that won't make it a crime under the law.