Veganism

[Thumpalumpacus]

It doesn't predict agreement because people can disagree for reasons unrelated to any given proposition being true or false, as mentioned, but it does suggest things like a/any rational moral agent a, supplied with facts b, could/must conclude c.  Ethicists might lean on concepts like perfectly rational agents when trying to describe basic system principles, despite there being no such example of such a thing in the human race, to point out the disparities in what we might rationally conclude compared to what we actually do.  People can be ignorant of facts, and it's not hard to see how not knowing something about a thing might lead to a different of conclusions about that thing if you were aware of that fact.  This is why vegan/vegetarian ethics often takes time to explain to people facts about animals that they may not, or many people do not, know.  Facts that seem to inform other moral conclusions we accept as true - making it an issue of consistency if nothing else.  That's the very question the op lead with.  What disparity in fact between animals and people makes it (seem) okay to eat animals, but not people?  Or would, if it's okay to eat animals even in light of those facts, somehow still not make it okay to eat people to whom those same facts apply.  

I'd probably agree that realism isn't the most practical system.  You have to think about shit.  You have to know shit.  It matters whether or not you have the facts correct.  You can be wrong.  Emotively, if we didn't know how we felt about something or had never thought of it before revulsion in the doing of it (or joy) is immediately instructive.  Subjectively, we do tend to know what we think about a thing, or can quickly arrive at a basic opinion.  Deontologically, we're good at lists and lists are succinct and practical.  Realism's real benefit is in accuracy.  Either accuracy in description of fact or accuracy of communication.  Realism says, for example, if what you mean when you say a thing is ad is a thing is yuck..then say yuck, because the thing being "bad" isn't really true.  Not as the reason for your opinion and not as the thing you're actually experiencing.  If you have an opinion on a thing, and it's not based in objective facts about that thing..then just communicate it directly as the opinion not based in fact that it is.  If, however, you think that there actually is something bad about assault, or actually is something good about helping people...that it's not just yummy to you or an opinion you cannot support with objective facts about assault and helping people...that's realism.

As far as evidence for moral truth as a phenomena, it's actually pretty strong.  From core principles of moral systems that are shared between individuals and cultures through time that allow us to recognize them -as- moral systems and even specific types of moral systems, to the neurology of moral consideration - where we see that we're genuinely trying to use the regions of our brain that deal in what we otherwise call fact (not exclusively! - there are neurological explanations for what realists might call emotivist, subjective, or relativistic moral failure), to the contents of the specific realist statements themselves.  If I wanted to explain why I think assault is bad I would inevitably talk about the physical and mental effects of assault on both parties, the historic consequences of permissive violence, though I may also just point at a victim and say "you look at that and figure it out yourself".  Think about it.  You probably wouldn't want to be quoted under a picture of some vicious domestic abuse saying "there's no evidence or fact that suggests doing this to a person is wrong".

... which is a fancy way of saying "we knows it's wrong" -- so why any philosophizing? No one wants to be murdered, or beaten to a pulp, or robbed of their life savings. Somehow some folks have carved out a career telling us why this shit ain't right? Well, Jesus please us, Gilligan, I'm so glad you came along so I could figure this out.

This sort of discussion is exactly why the ivory tower is seen as the domain of the oblivious.

Now, veganism itself -- you've either decided animals are lesser than you, and therefore edible, or you've decided they're equal to you and should be regarded the same respects. You don't need a goddamned four-year degree.

I would prefer not being eaten. I don't think a brown bear or great white gives a shit.