Different folks fall under the "zoo" header for various reasons, I'm only going to talk about what I am familiar with and let others explain their perspectives.
I have always had an affinity for non-humans and when I reached puberty it seemed perfectly normal to me to explore that. I happened to live on a farm at the time, so I explored with the animals I knew and was familiar with. At the time, never having had a relationship with a human I just assumed that when I found a girlfriend I'd forget about it, but that was not the case for me. I had a number of variously unsuccessful relationships where I would try to forget my feelings about non-humans, but I was never able to be satisfied nor happy in such relationships.
After about a decade of trying and failing to make human relationships work, but inevitably always going back to or thinking about non-humans I spent some time soul searching and I started to accept that it wasn't a fault of anyone else, nor was it my fault, just an incompatibility and I stopped trying to shoehorn myself into relationships where I would never be happy (and ultimately, make my partner unhappy).
It's been many decades that I've sought relationships with non-humans. In that time I have outlived a few partners and deal with the sorrow and loss, but I always end up with another dog because sex or not, life is better with a dog than without.
I believe the current science suggests that for many zoos this seems to be something baked into us before puberty, I feel that is the case with me. If I could change it I certainly would because while I love my doggy partners deeply, it sure would be nice to grow old with a loved one instead of loving and losing them over and over. It's a blessing and a curse, to love another so much. Its certainly never relented.