yytz,
Bestiality can be consensual too. In fact it can be initiated by the animal (there is a Gemara in Kesubos that describes such an occurence and my understanding is that this is not uncommon [relatively speaking of course]).
That being said, I can see a distinction between the two. However, I do not believe that distinction (the greater potential for abuse) is the reason why we treat the two differently. The real reason is that we find it perverse, disgusting and morally deplorable.