I'll be in the minority here, but I generally am not personally comfortable with male (or female) penetration of animals. I don't judge anyone else who is though. This is coming from the perspective of a non-zoo / voyeur and a zoo-wife.
From my perspective, taking a more passive role and being fucked or licked by an animal is one thing. Even if the animal is perhaps guided or assisted. Obviously, from what I have read, animals are trained and are not normally just going to mount and go to town. So I get that there's more going on.
I personally struggle with when there's more of an active human role involved. Even when it comes to giving oral to the animal. To me, these are creatures that are not necessarily as capable of consent. While they may be trained and may allow things, I just find it easier to understand and accept when the animal is doing things more on it's own accord, ie mounting, humping, licking, etc.
Reading the rules, I believe this aligns with the whole idea not to coax animals into doing something sexual, such as using foods, spreads, etc. I'm generally against when people self-masturbate using the animal's penis, etc. The power dynamic is just too different and I believe we have a responsibility to the animals we love.
To reiterate, I am not saying taking an active role is wrong. Not in the slightest. Just sharing my perspective regarding the topic. To each their own. I'm just happy to share a space with similar like-minded people who are open to discussing the topic and sharing their opinions. We're all different, yet so alike.