Thank you for bringing this up.
Let's set aside the fact that animals have been proven to show human emotions such as fear, empathy, love. Let's also set aside the fact that it will be rare to find instances of a higher-order organism trapping a lower-order organism and 'harvesting' it for food over and over again.
Do you agree that animals are sentient beings who fear death? If so, I hope you will also agree that animals would not want to die in order to become someone's food. That's where humans, with all our higher faculties of thought and reasoning, need to apply our standards of ethics.
When it comes to matters such as homosexuality,
promiscuity (for lack of a better word for multiple sexual partners) etc., if any person were to point out that since these activities occur in the animal kingdom and hence they are natural, plenty of people (especially thiests) are quick to put them down by retorting "Humans are above animals" "And that's exactly why humans are not animals" "Humans have more mental capacity than animals"...you get the point. We are quick to apply "human standards of morality" to these activities.
I find it amusing how our superior mental capacities are then not used to also argue against the killing of animals, when we are perfectly capable of growing our food in such a manner that no human will die from hunger or even suffer from malnutritition thanks to human advancement in agricultural technology.
Our standards of morality persuade us to not kill or hurt other humans, but just because other sets of organisms who do not talk, walk or look like us, suddenly should our standards of morality/compassion stop being applicable? I find it pretty hypocritical when a human feels the pain of another human dying in another part of the world, but doesn't consider the pain of killing an animal in front of his eyes. As if our compassion is supposed to be restricted within a certain domain.
And ofcourse there's another debate to be had about energy's input/output ratio in the meat production industry.
PS: If people feel it is okay for human compassion to be restricted within a certain domain, then continuing on in the same vein, I don't see why people should get outraged when one group of humans (ethnic, religious, whatever) slaughter another group of humans. The first group just chooses to apply an even more restrictive domain on its standards of morality/compassion.