It's an interesting question. I haven't done the study necessary. I certainly wasn't prepared to do the study necessary at this point to show whether there is any evidence to suggest that those who commit bestiality go on to commit other sexual offences.
If you look at what the prohibition order does, you'll see that it's designed to stop sexual offenders from being in situations where they can commit further offences. It keeps them away from children. It keeps them out of positions of responsibility, and so on.
I don't have the evidence to answer whether or not that's true of those who perform bestiality simpliciter. To be clear, where they perform it in the presence of children, that is sexual offending. I have no issue with that and the code already deals with that. Whether or not there's the proof to show that it's necessary, I unfortunately don't know.