(I'm not a expert, I gladly accept respecftul debate, and new info. Feel free to join the conversation)
Let's get a little imaginative here...
Evolution is the sum of genetics + enviroment
Now we need to make a few (but heavy) assumptions:
1) There is a group of genes that makes us think about zoo (there isn't)
2) that genes served in the past to synchronize the mating time for the human species (monkey man sees animals mating, monkey man wants to mate too), but wasn't necessarily attraction to animals, just another way to mate at the correct season (the genes hardly works like this, but that's how we can explain how we develop hereditary behaviours). Behavioural genes aren't my strong suit
We have one of the more complex structures in the universe: our brains. At some point, some individuals develop a change, a change that gives us our third assumption:
3) Now we have in our dna something that makes us wanting to mate *with* the animals and no *because*
That could be one explanation, but it is reaaaaaally hard to demonstrate it, and we have found veeeery few genes attached directly to ancestral behaviours
Second explanation: culture
The humans (and maybe very few other animasl) are the only ones who can be demonstrate that we have a culture. These gives us unique phenomena:
-Besides attraction we have love
-Besides aggresion we have violence
Both are concepts that are shaped by culture. Love and violence meanings vary from one group to another group of people
At some point, we took simple concepts (it's good to care for the animals, I love them), but we make complex individual relationships (jeez! dogs mate strong and fast, and... Wou, the cum. Now I feel hot how they mate) And we create complex ways to understand ourselves and the world (ok, I found really hot the idea of I mating with the dog).
That's just my ramble, nothing proven or read or papers, buuuut, if the discussion is good, this could be a good starting point to share the info
(non-native english writer here, feel free to correct me or guide to improve my writing

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