This is not typically a decision. You are born like it or you develop this way during puberty, but there is no point at which you would consciously decide that from now on, I am going to be zoo exclusive.
If this was the case, then you could go back to being into humans at any point, which is not the case when being zoo exclusive.
Being zoo exclusive is basically that your sexual orientation is towards animals and you do not find people sexually attractive. Changing your sexual orientation is not something people typically can do without severe psychological trauma.
Here is a quick set of definitions:
"Matthews, the author of The Horseman (1994), explains that there are two general classes of people who have sex with animals. There are the “bestialists” who have one or a few sexual contacts with an animal, or they use animals when a more “ normal” outlet is not available. The second class of people are the “zoophiles.” These are the people who prefer an animal as a sex partner, often forming deep emotional relationships with it (Matthews, 1994)"
...
" ...I am zoo exclusive and the very thought of having sex with a human disgusts me. Ask a homosexual if he wants to have sex with someone of the opposite gender."
Hani Miletski - Understanding bestiality and zoophilia.
I would have to disagree with it not being a decision unless trauma is involved. I've been a zooromantic (first and foremost) zoosexual most of my life, but not all of it.
I started out with a couple crushes on schoolgirls like most kids do, before completely falling deeply in love with the shepherd-malamute male next door years later.
We kissed and cuddled for long periods whenever we could be together privately. Just to be able to touch him and take in his scent was such a fulfilling experience that it would take everything in me to not tear up in front of the humans nearby from all the love and joy I was feeling with him.
Getting to this point and beyond (to sex) was a very organic process. Started out as basic friendship, to spiritual connection/awakening that caused me to feel more canine than human but out of pure love and just this vibration that flowed through me. It's hard to describe other than to call it a "spiritual awakening" with wolves and canines in general, although I didn't really think of it as "spiritual" anything back then and had no belief system or significant spiritual influences, from humans, at all back then, although this experience kind of gave one to me in a sense not based in anything I learned, it was just an intuitive process I guess you could say. This is typically where I lose most people talking about this online. Regardless, it was a deeply emotional and devotional experience for me, being with him, always longing for him and this is what caused me my first relationship to be an intensely zooromantic one.
The fact that this beautiful canine next door looked pretty wolfy just added to it. And his scent...oh my God so beyond intoxicating.
By this point I was already absolutely 100% zoo exclusive, before even the thought of sex entered the picture. The more I loved him and wolves the less I could even tolerate humans let alone desire intimacy with them! Mostly because of how humans viewed and treated other species especially wolves and dogs.
He and wolves became my universe and my everything. I would've laid down my life for him without a second thought to protect him. I virtually worshipped his presence, it was intense emotionally and spiritually.
Then 1-2 years later, sex entered the picture (after we both discovered it was possible for us). No trauma that I'm aware of at any point. Not even internet access or anything else to really influence me zooromantically or zoosexually back then. We were on our own from the start. Yet it never felt wrong, never felt shame of any kind. Fear of humans finding out, yes of course. But I was so incredibly happy, honored and proud to be his mate.
(Edited for typos)