My dog is all about routine.
Your dog doesn't spend its entire life in a box, you take it to the vet when it gets sick or injured, and you take it outside to poop so that it doesn't have to live in its own feces. You presumably feed it food intended to taste good and be healthy for dogs.
I actually DO think seriously about the rest of your argument, but before I seriously respond to it I want to be clear that the comparison with your dog is almost completely irrelevant, and distracts from the issue.
I don't know how the "net suffering" works out. I don't know whether, if given a choice somehow, animals in a factory would choose to die or to live, and how much they'd care. But I know that, at least with mammals, chances are the neural pathways are similar enough to humans that I can make some assumptions.
Self reported happiness levels suggest that human happiness DOES adjust to new circumstances. People have a baseline level they usually return to, with dramatic events mostly causing temporary spikes. I've also read that people tend to be less happy when they have to agonize over available choices. So given that factory-farmed animals have pretty much one constant state of existence, and they are probably incapable of imagining anything better, I AM willing to grant the possibility (perhaps even likelihood) that their existence isn't a constant stream of torture, but rather a dull, aching pointlessness. Plenty of humans live in conditions just as bad and, however bad it gets, opt not to kill themselves.
But I don't think "opt not to kill themselves" is a useful metric for determining a good moral framework. Human slaves may have adapted to their situation and made the best of it. That doesn't mean that "breed a race of slaves that doesn't know any better" is a good thing. I'm of the opinion that if we've decided that we have moral justification for producing infinite creatures with abysmal but barely livable lives, then it's far more likely that we have a made a mistake than devised a useful system of morality.
For the record, information on factory farming (as well as how pointless most "humane" labels are) can be found here: http://www.farmsanctuary.org/issues/factoryfarming/
Your use of "pointlessness" makes me think of something. Saying that someone's life has a point (purpose) usually means they have some long-term goal that they work toward, which implies thinking about future possibilities and developing abstractions like "purpose". Not to say that the animals most people eat can't think into the future, but if they do live "in the moment" to a much greater extent than humans, doesn't that greatly reduce the cruelty argument? I think a sufficiently unintelligent human would be happy to sit and...
(Note: I wasn't quite sure whether this warranted a high level post or just a discussion. I haven't made a high level post yet, and wasn't entirely sure what the requirements are. For now I made it a discussion, but I'd like some feedback on that)
I've been somewhat surprised by the lack of many threads on Less Wrong dealing with vegetarianism, either for or against. Is there some near-universally accepted-but-unspoken philosophy here, or is it just not something people think of much? I was particularly taken aback by the Newtonmas invitation not even mentioning a vegetarian option. If a bunch of hyper-rationalists aren't even thinking about it, then either something is pretty wrong with my thinking or theirs.
I'm not going to go through all the arguments in detail here, but I'll list the basic ideas. If you've read "Diet for a Small Planet" or are otherwise aware of the specifics, and have counterarguments, feel free to object. If you haven't, I consider reading it (or something similar) a prerequisite for making a decision about whether you eat meat, just as reading the sequences is important to have meaningful discussion on this site.
The issues:
1. "It's cruel to animals." Factory farming is cruel on a massive scale, beyond what we find in nature. Even if animal suffering has only 1% the weight of a humans, there's enough multiplying going on that you can't just ignore it. I haven't precisely clarified my ethics in a way that avoids the Repugnant Conclusion (I've been vaguely describing myself as a "Preference Utilitarian" but I confess that I haven't fully explored the ramifications of it), but it seems to me that if you're not okay with breeding a subservient, less intelligent species of humans for slave labor and consumption, you shouldn't be okay with how we treat animals. I don't think intelligence gives humans any additional intrinsic value, and I don't think most humans use their intelligence to contribute to the universe on a scale meaningful enough to make a binary distinction between the instrumental value of the average human vs the average cow.
2. "It's bad for humans." The scale on which we eat meat is demonstrably unhealthy, wasteful and recent (arising in Western culture in the last hundred years). The way Westerners eat in general is unhealthy and meat is just a part of that, but it's a significant factor.
3. "It's bad for the environment (which is bad for both human and non-human animals)." Massive amounts of cows require massive amounts of grain, which require unsustainable agriculture which damages the soil. The cows themselves are a major pollution. (Edit: removed an attention grabbing fact that may or may not have been strictly true but I'm not currently prepared to defend)
Now, there are some legitimate counterarguments against strict vegetarianism. It's not necessary to be a pure vegetarian for health or environmental reasons. I do not object to free range farms that provide their animals with a decent life and painless death. I am fine with hunting. (In fact, until a super-AI somehow rewrites the rules of the ecosystem, hunting certain animals is necessary since humans have eliminated the natural predators). On top of all that, animal cruelty is only one of a million problems facing the world, factoring farming is only one of its causes, and dealing with it takes effort. You could be spending that effort dealing with one of the other 999,999 kinds of injustice that the world faces. And if that is your choice, after having given serious consideration to the issue, I understand.
I actually eat meat approximately once a month, for each of the above reasons. Western Society makes it difficult to live perfectly, and once-a-month turns out to be approximately how often I fail to live up to my ideals. My end goal for food consumption is to derive my meat, eggs and dairy products from ethical sources, after which I'll consider it "good enough" (i.e. diminishing returns of effort vs improving-the-world) and move on to another area of self improvement.