I don't think there's a straightforward answer when it comes to animals doing the penetrating. The animal isn't being physically harmed (or if they are, it's obviously wrong) and they're not going to suffer psychological trauma in the way humans do (highly intelligent animals like apes might be an exception to some small degree, though again, I doubt any psychological effects would come close to the profound PTSD experienced by humans). That said, informed consent is still going to be an issue. There's also concern about zoonotic diseases, but that opens the whole "well what about humans spreading disease to each other, which is much more likely?" can of worms.
Humans penetrating animals, however, is straight up immoral.