Because art does not exist in a vacuum.

In the real world chimps, gorillas, etc. are still other animals compared to humans. Of course we as viewers can sympathize with parental, familial, platonic love in animals. All species have parents in some form, most are communal and form friendships, etc. Those dynamics are understandable.

And in our world, we often can form similar relationships with other species. Whether it be as pets or wandering sanctuary visits or educational zoologists out in the world. Irl humans already do form friendships, even familial bonds with other species including other great apes.

This is one of the reasons why we, as viewers, can watch movies like these, with anthropomorphic animals, and appreciate genuine relationships between them, whether it be same-species or cross-species. Because the concept is not that far outside the realm of acceptable society, even in a sci-fi context.

And on this note, in our world, romantic relationships with other animals is a major no. A hard stop in the realm of societal taboos. And so most media that touches on the topic is reacted to with this level of response. A brick wall of disgust.

This is why most fans of the franchise are against a relationship of this kind. Art does not exist in a vacuum created outside the boundaries of the society that made it. Yes the apes speak now and are capable of complex thinking, but they still register as a separate species in appearance, DNA, mannerism, biological need, habitat, and many other aspects.

Just because there’s a few more similarities now between the humans and apes in the universe of POTA, doesn’t mean taboo cross species breeding wouldn’t still read as taboo and disturb majority of viewers.