Vegetarianism feels to me kind of like an issue where bike shed arguments are rampant because everyone eats and everyone wants to be moral and "how hard can it be to decide what to have for dinner"? So everyone has an opinion... and feels entitled to speechify about it :-)
In the meantime I suspect that if there is a correct answer here (like there is some kind of diet that simultaneously optimizes moral, health, economic, social, and deliciousness issues) it probably doesn't exist yet and could take decades of research to properly nail down. Maybe some sort of vegan school would actually make sense, rather than just being a fun joke? But if a vegan school makes sense then non-experts who talk about the subject are likely to be spreading BS...
And in the meantime there's this huge number of moral prerequisites to being right about moral vegetarianism that I'm just not sure about. Despite being opportunistic omnivores rather than obligate carnivores, humans are the apex predator of the planet. (This isn't quite as weird as it seems, because for a while grizzly bears occupied much the same niche in north america.) As far as I can tell, this makes humans (not otters or sea urchins) the ultimate de facto keystone species of the planet. We sometimes eat sea urchins and wear otters -- hence regulating the size of such populations, thereby determining the functional modes of the global ecology.
Supposing that there was something (vampires?) that preyed on humans and that we had absolutely no pragmatic recourse against these new apex predators (imagine saying "don't eat me!" to a nano-wielding fusion-powered super-intelligence that likes eating humans for some reason) what policy for the consumption of humans would we hope that they adopted? Do we want them to start eating human shaped tofu and relegate humans to zoos and textbooks? Do we want them to bioengineer brainless human-flavored vat steaks and let wildtype humans go extinct? Do we want them to give us adequate human lives and then eat us in our 40's before our flavor goes off? Do we just want them to figure out what maximizes the happiness/virtue/number of vampires (maybe re-engineering themselves to eat algea and wheat and putting every calorie that strikes the planet into the production of this food)? Do we want them to manage wild human populations such that global biomass and biodiversity is maximized? Do we want them to domesticate us via manipulation of our genetics into a variety of types designed to serve specific ends within their economy? Maybe they could offer us some kind of "minotaur feeding treaty" where we managed some of these issues ourselves in exchange for super-trinkets? Lots of those suggestions feel to me like they have good points and bad points... so framing the question this way helps me see that I don't really know what kind of global dietary regime that I really want to bring about with my food choices as an apex predator. I am confused.
In the meantime, I grew up on a hobby farm, and we gave the cows we planned to eat names like "Burger", "Choice", "Delicious", etc so that we wouldn't get too emotionally attached to them. Those cows had OK lives, and they were pretty tasty. Later, in college, I lived in a vegetarian house for a year and while I had nothing in particular against the diet (and found out that nutritional yeast is really yummy with pasta and popcorn) it seemed to me that there was a correlation between the degree to which vegetarians are hard core and the degree to which they seemed prone to cognitive dissonance and generally bad arguments. No one could really agree on the details of what should or shouldn't be eaten, which is what you would expect if no one had collected enough evidence to honestly determine a real answer on this subject.
And then on the other hand (am I up to three of four hands at this point?) it seems that dietary restrictions are a very common part of many religious communities. No beef for Hindus, no bacteria or roots or meat for Jains, no pork for Muslims, no beans or meat for Pythagoreans, and kosher laws are notoriously complex... From the outside, all of this stuff looks to me like a memetic retention mechanism for preventing people captured by a belief system from breaking bread with non-believers and thereby becoming positively disposed towards alien beliefs. If these social/signaling/affiliation processes are the real issue, then my personal meta-strategic goal is to be able to prepare food under any set of restrictions so I can invite awesome people over for dinner no matter which conducts they've chosen to play under. Also, I should be willing to eat nearly anything someone else serves me. Anytime someone I'm not intimately familiar with offers me food, whether its baby seal soup or sprouted spelt granola, the response I'd like to give them is "Thank you, that was delicious!"
I think the Newtonmas invitation was probably made in roughly this last spirit, where a group of people who know each other quite well had a party and the host solicitously offered food roughly consistent with the paleo diet (which a number of expected guests probably subscribe to), and which signals (if anything) a nominal allegiance to personal health maximization in light of a rational recognition of our evolutionary history. Given budgetary constraints and knowledge of the host and guests, it was a socially reasonable thing to do.
Which brings us back, perhaps, to the original question: If that is how menu selection worked out in that social group, does it imply some gross moral or intellectual failure on the part of the entire group? Maybe it does if some particular dietary choice seems "obviously right" but if the right answer isn't clear (which is my own sense) then I'd guess not.
There are Buddhists who are vegetarians, but who also believe that accepting hospitality is more important than not eating meat.
(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.