Consider the following thesis:
(*) It is permissible to kill an animal primarily for food only in special circumstances such as where other food is not reasonably available, affordable or sufficiently nutritious.
According to (*), there is a presumption against killing animals for food. If (*) is true, then everyday meal situations for middle and upper class people in Western countries are not going to count as "special circumstances"--other nutritional sources are available to us. I shall also take it that if it is impermissible to kill animals for food apart from special circumstances, it is also impermissible to buy parts or wholes of animals killed for food, again apart from special circumstances.
I think (*) is false. I also think that if (*) is false, then theism is probably true. In this post I will do two things. First, I will summarize an argument for (*) by Andrew Tardiff (published in Faith and Philosophy, some time in the 90's, I think). Second, I will show that some theists have a way out of this argument. Third, I will suggest that probably this is the only way out, and hence if (*) is false, then probably theism is true.
Tardiff argues that (*) is true because it is bad to destroy highly organized entities without proportionate reason. Animals, especially higher animals, are highly organized. Our survival would count as a proportionate reason for destroying an animal. But our gustatory preferences do not count as a proportionate reason, nor do the minor inconveniences of preparing nutritious vegetarian meals count. Special circumstances that do not obtain for us in the affluent West these days would be needed to justify eating animals. Tardiff is a Catholic, and neatly harmonizes his view with the Christian tradition which permits eating animals by saying that eating animals is not intrinsically wrong, and indeed was permissible in past centuries where it was more difficult to get adequate nutrition apart from animals. (I am going by memory here, but I think I'm faithful to Andy's paper.)
But there is an answer available. Suppose that non-human animals live for our sake and were made for us to eat. Certainly, it seems natural for us to eat animals, and if God has coordinated all of nature, it would be plausible that he would have made it natural for the animals to be eaten. On such a view, it is a telos of some animals' lives to be food for humans. Such a view is coherent, and there are aspects of the Christian tradition that imply it. Unless one has disproved this kind of a hierarchy of life view, one has not shown that it is wrong to eat meat under ordinary circumstances. (I can't remember what, if anything, the paper says about this sort of an answer.) In fact, on a view like this, not only is there no presumption for vegeterianism, but there is a presumption for eating meat--if it is one of the telê of the life of a deer that it provide humans with food, we have prima facie reason to fulfill that telos.
And I think something like this is the only objection available. For if it is not a telos of a boar to be eaten by humans, then to kill a boar for human food, apart from special circumstances, is to do a harm to the boar--its death is contrary to its telê on this view--without proportionate reason. Thus, unless it is a telos of at least some animals to serve us, even to the point of our eating them, (*) is true. Granted, there are also some consequentialist considerations--maybe there would be fewer individuals of some species if we didn't raise them for food--but these probably aren't going to amount to a good defense of the negation of (*).
Thus, probably, if (*) is false, then it is a telos of some animals to be eaten. But, probably, the only way that the benefit of one species can be the telos of the functioning of another is in cases of reciprocity, e.g., in symbiosis, unless that telos has a source in the plan of some agent who arranged the system. But reciprocity need not hold for humans to be permitted to eat animals under ordinary circumstances. There may be some reciprocity with domestic animals, but I think we should likewise deny (*) in the case of wild animals. It seems implausible that there should be a biologically-mandated telos in deer and boars that they be eaten by humans. Thus, the telos in question has a source outside the biological system.
Moreover, a merely extrinsic telos, e.g., a purpose that an agent has for the entity, will not justify the killing of animals for food. For instance, that we bred some set of animals for food does make the feeding of us be an extrinsic telos of them, but is not itself sufficient to justify our killing them, because an extrinsic telos of an animal does not affect what is good or bad for the animal. (That my parents had some purpose for my life does not constitute it as being good for me to fulfill that purpose, though I may have reasons of gratitude or prudence to fullfill that purpose.) There is no presumption in favor of acting in accordance with an extrinsic telos: I can equally well use a sword for fighting as for a plowshare.
Thus, in order to justify our ordinarily eating animals, their telos to be eaten by us needs to be an intrinsic telos. But the only account available of a non-biological source of intrinsic telê is the account that God, who is the Ground of Being, can determine which essences, and hence which telê, are instantiated.
Thus, probably, if (*) is false, God exists.
Hence, most middle and upper class American atheists should be, for consistency's sake, vegetarian. A number, of course, are.