Again. it's about the quality of the argument - it's not enough to simply say that something is bad, you have to actually say why.
I suggest that we need a different argument or reason to justify ethical pronouncements about human/animal behaviour than the ones we use to pronounce on human/human behaviour, which is why all the stuff about slavery etc is utterly irrelevant here as far as I'm concerned.
I also note that arguments which fail to make the distinction between the two classes of behaviour and ethics frequently run the risk of ignoring what is special or particular about humanity and can (and I'm not saying that anyone here is explicitly or deliberately doing this) end up being anti-human.
And as I've pointed out before, the very fact that this thread, ostensibly about human health, has gone off on to this huge tangent about the ethics of killing animals for meat suggests that many people don't really care that much about the issue of human health - it's easier to simply dismiss all those meat eaters as bad people who, presumably, deserve to be unhealthy and die early.