"Personally, I have not heard a single realistic, convincing, non-religious argument for the use of non-human animals (for food, clothing, research, entertainment, etc) by humans. In fact, most of the arguments given for using animals (lower intelligence, rankings of usefullness to humans, etc) apply to marginal humans also. I'm curious what the users of this forum think."
If we apply the same standards to all animals, including humans, then we are not guilty of speciesism. It is not speciesism to treat lower animals in the same way as we treat human criminals. Simply put: lower animals do not respect our rights, so they give up any logical basis for expecting us to respect their rights. We can kill them, capture them, eat them, experiment on them, keep them as slaves, etc. The only principle limiting what we can do to them is this: we must not, by our behavior, give other, non-criminal beings reasoned grounds for perceiving a threat to their own rights. That means we can shoot a cougar that attacks our livestock, but we cannot capture it and torture it, because by doing the latter, we would give other persons reasonable grounds for thinking that we are criminally insane ourselves. (It is not an accident that serial murderers, as children, almost invariably have a history of torturing small animals.)
As far as eating meat from executed human criminals goes, cannibalism is a dangerous practice, because any diseases or parasites that the source of the meat had will be potentially transferrable to any individual of the same species that eats the meat. Eating human meat, by a human, falls pretty much into the same category as playing with a live hand grenade. Hence if executed murderers were butchered out and the resulting cuts of meat sold in grocery stores, I doubt that there would be much demand for them by other humans, though I suppose they could be included as an ingredient in dog food.
In any case, I don't see even a shred of "speciesism" in using lower animals as food, keeping them as pets, or any of the other things that you seem to be concerned about. Lower animals, by their nature, are incapable of respecting our rights, so we are not obligated to respect their rights.
And that's all there is to that story.