You keep using these abstract categories "sentience", "voluntary". Look, my dog is more valuable to me than some people. Morality and ethics are often a serious of concentric and hierarchical relationships the way I see it. Maybe there are rules, but even if they would always be the same they might have to be applied differently in different circumstances. I think these are difficult questions and it seems you are blinding yourself to many aspect of what makes or does not make something right or wrong.
The abolishment of slavery was not really that gradual. But it's all irrelevant: the point is that we still have laws against it, and that we did not evolve to stop treating people as our inferiors by making them slaves, or something similar. It is also not a correct analogy. Slavery is not binary in social reality, only in the way we talk about it as a concept, or idea. There is chattel slavery, ...-slavery, wage-slavery (arguably). A similar comparison with eating meat/ not eating meat would be if we went from a society of slavery to a communist paradise. We didn't, instead we still have hierarchical societies where the most extreme forms of dependence (those we happen to legally classify as "slavery") etc are forbidden by law.
What does free will have to do with my argument? Whether an act is voluntary or not can be relevant for morality yes, but I think other things are also relevant. I am acted upon against my will all the time. Just yesterday there were a bunch of hoodlums playing loud hip-hop music on the beach near my house. When I walk around in certain parts of Stockholm I am forced to see incredibly ugly buildings. I did not choose any of this, is it immoral? Perhaps we should ban all these things because I didn't choose them? If I enter a contract to voluntarily allow someone to destroy my grave after I die, or to fight in a gladiatorial game, is that moral? If I have ... with my sister, and she wants to - is it moral? Voluntarism is an insufficient basis for morality. Sometimes voluntarism has nothing at all to do with morality.
No, my last question is not unrelated. You think it is unrelated because you believe that physical suffering and voluntarism are the only two criteria for what makes something moral or immoral (at least that I've seen you put forward). The last question is in fact very important, because its aims to set the boundaries for what morality is. If you can't answer that question, you have no convincing moral claims at all the way I see it.
What makes one principle better than another? The essence of morality is saying something is better than something else, somehow. I say that the tastiness, sense of meaning and whole set of cultural values that an individual and society can get from eating meat is legitimate, but I admit that there are moral downsides to it - as there are to practically everything. I just think the alternative you present of a vegetarian "utopia" would be morally worse, overall.
Why does physical pain mean that there is moral equivalence between animals and humans? Why is your assumption that such equivalence exists superior to mine that it does not?
