Why all the past animals want to fuck him?
Unironically, I think a lot of domesticated pets don't clock in the species barrier if they weren't socialised properly. I've atleast seen it in birds, where they think some parts of humans look similar enough to another bird that they believe a head of hair to be a mating partner. In the wild I don't think animals really understand romance as a concept, so it really comes down to their genetics not having developed a way to filter out things that look similar enough to them. It's the same basis for nest rearing birds and why cuckoo's are genetically successful despite carving put a niche that actively harms other species niches by killing offspring.
So in summary, in addition to being genetically predisposed to being unable to clock in species barriers, they are susceptible to being raised to choose a human who feeds them as a more viable partner than another member of their species if they don't get socialised properly.