i only needed to read the first two posts on this page to understand the entirety of this topic and i'm assuming i need not know any more
bestiality isn't super-cool to most people and that's not changing; consent is typically only considered valid between creatures capable of fully understanding the choice they are making, which we do not consider to be the case for any other living species, because they cannot understand complex interpersonal human relationships or loveual contact past their basic mental function to reproduce and possibly enjoy getting their genitals stimulated
it's not in the permission being given as much as it is in the ability to fully understand the decision whilst making it, which is why paedophilia is also a problem.