So, here's what I wonder about this. I think most people who think about it can agree that a lot of factory farming conditions are terrible. Cramped quarters, lack of outdoor time, space, exercise, poor food, etc. So working toward eliminating those problems seems incontrovertibly good to me.
However, let's say hypothetically we're only talking about farms run with a focus on animal welfare. (And not just giving animals 10 square feet of 'pasture' so they can say they're free range, but really.) So the issues of poor living conditions are removed, and we're just talking about killing animals for meat. If people stopped eating meat altogether, most farm animals simply wouldn't be born in the first place. Farmers aren't going to raise animals if they can't eventually earn money from them, obviously, and most of these animals wouldn't exist in large numbers in the wild. So what I'm wondering is, for a farm animal is it not possible that a decent life followed by being slaughtered for food might be superior to no life at all?
Obviously if we think in terms of humans, that would be horrific; we consider killing humans unnecessarily an absolute moral wrong. If someone decided they would have a child, but only if they could murder them after some period of time, that's obviously monstrous. I'm just not sure that that analogy applies to farm animals who certainly are living creatures who feel pain, but don't have the same understanding of the world and their place in it that humans do. To me that's the rational reason why the the idea of killing humans could be considered differently from killing other animals. That said, I expect the real reason we instinctively feel that killing humans is wrong is more an evolutionary drive to protect our own species.
I do grant that it's uncomfortable to think about when the analogy to humans is drawn. For some I'm sure it's uncomfortable even without consciously making that connection. I can absolutely understand why someone would therefore conclude it's morally wrong to use animals for food. I'm just not sure I agree.