Jonathan Ichikawa has an interesting argument for vegetarianism. It would be immoral to have a party where we throw human parts around just for the fun of it, parts from people grown specifically so that we can have enjoyment in doing exactly that. Jonathan suggests that it would be wrong to attend the party even if such attendance doesn't cause the practice or fund it in any way. He says this is because it supports it. I'm not sure that's the real reason it's wrong. I agree with Richard's comment that mere participation in an evil practice is wrong. I don't think it's supporting it that's the issue. It might be ok to support an evil practice as long as one's reason for doing so is not to use that evil practice as a means to your end but rather that the support is an unintended side-effect that your end doesn't itself rely on. But in this case that's not what's going on, since you're actively participating in the practice. That's what seems to me to be wrong about it. Is the same true for eating animal meat produced in factory farms? The argument assumes that animals in factory farms really are treated immorally, but that's something I agree with.
One way to distinguish between the two cases is if you have a basic distinction between our moral obligations to animals and our moral obligations to humans. Some people, for instance, think we have moral relationships with humans as our fellow members of a moral community that we don't have with animals. This doesn't mean we have no moral obligations toward animals, but the fact that they're not in the moral community with us in terms of being moral agents themselves is, on some ethical theories, reason for taking them to have moral properties of a different order. Perhaps this is not having rights of the same sort. For example, it's easier for our obligations to animals to be outweighed by other considerations than it is for our obligations to humans, or perhaps our obligations to humans can never be outweighed, while our obligations to animals can.
I don't myself think this is sufficient, because I think attending such a party with animal parts is immoral (even if you don't pay to attend). I do think it's much less wrong than with people parts, but it's wrong. The reason is not that you're funding the killing of animals for mere entertainment (although if it's also that, it might add to its wrongness). It's also not that it's merely supporting such a practice. You're engaging in the very entertainment goal that constitutes the evil practice. You're not merely supporting it, and I think engaging in it without supporting it is wrong. Suppose, for example, that you were able to sneak in, take an animal part or two off to your own private room with some friends, and engage in your own game of catch for enjoyment. I don't see how that provides any support for the practice. You even detract from it by stealing some of their animal parts without ever engaging with those who are doing it in any positive way. But you participate in an important way in the behavior that constitutes the practice.
So I don't think this is the right way for those who defend meat-eating to respond to Jonathan's argument. Merely distinguishing between the moral status of humans and animals won't be sufficient if the same practice with animal parts is wrong, and it's wrong merely to participate in it. If eating meat is analogous to playing games with it, then merely eating it is wrong. That's participating in it. It doesn't matter if you aren't providing any noticeable (or even any) support to the practice that will help it continue or fund those who do it. Even if you stole all the meat you eat, you'd be participating. So I think defending omnivorous practices in a world where most meat comes from factory meat-market is going to require finding some disanalogy between the case of eating meat and the case of throwing animal meat around for entertainment.
One difference that comes to mind immediately is easier to see when you think of a similar practice with food that doesn't come from animals. I know a group that regularly plays what it calls Food Games. At retreats and things of that nature, they will use food for fun group games. Sometimes it does involve eating some of it, but a lot of it is just for messy group activities, and even the eating games don't use all the food up. I've always been disgusted by this practice. It just strikes me as wasteful behavior. Wasting good food for mere entertainment value is wrong. I struggle enough with the practice of cooking more food than you'll eat and not being able to finish the leftovers before they go bad. We do that, and I think it's immoral enough to waste food unintentionally. Isn't it much worse if you're doing it deliberately? It's true that it's not a complete waste, because some gets eaten, and what doesn't is at least used for fun, but the entertainment value isn't a sufficient reason to waste food in those proportions.
I think once we see this, it becomes much less clear that the problem with throwing food around is that we're using the animal parts as a means to an end. The problem seems to me to be that it's using it for a particularly frivolous end, not that it's using those dead animals as a means to an end. Once you've distinguished between the kind of moral status animals have (which may not prohibit using animals as a means to an end) and the kind of moral status humans have (which may well prohibit using people as a means to an end), I think that opens up a lot of room for some means-to-end use of animal parts. Isn't deriving sustenance from them a much better use of such animal parts?
I realize that vegetarians can be relatively healthy and survive plenty long without meat, but I've never been convinced by the claim that it's healthier to be a vegetarian. Doctors and biologists who don't seem to have an axe to grind generally tell us that our ideal diet is lean meat with supplementation from other sources. The only people I've ever seen claim otherwise are vegetarian apologists. Some people even say it's best to have the kind of animal proteins that are only found in meats (only some of which are even in eggs and milk). It's possible that some people have even more reason to need animal proteins for specific health conditions. But the key point is this. Throwing animal parts around for fun is what's wrong, and taking part in that activity is taking part in the wrong. Eating meat is not wrong in itself (I'm convinced; I'm granting here only that factory farming is wrong, not that eating meat in other circumstances is wrong). Eating meat, then, is benefiting from others' wrongdoing. But I don't think there's anything wrong with that.
Now none of this provides any reason to accept the status quo. Meat eaters ought to oppose factory farming. I'm not convinced that boycotts are the best way to convince businesses to change their practices anyway, so I don't think vegetarians are having much effect on the factory farmers by not eating meat. I do think more people ought to make this issue a priority, particularly those who eat meat who don't approve of factory farming. It may also be that certain animals are treated more cruelly, enough moreso that it's worth avoiding those (e.g. maybe refraining from eating veal), but I'm not convinced of that at this point. But I don't think it's the same sort of participation issue as throwing meat around for fun. It's the mere frivolity of that case that makes me willing to say merely participating in the practice is wrong, even if you aren't funding it or supporting it in any other way. Since that isn't true of eating meat, I don't see how the analogy is going to work as easily.