As a person who was vegetarian for 8 years, a vegan for one, and now a happy meat eater, also a lifelong gardener, I think both sides of this argument are being intellectually dishonest.
Number one, if you take the time to live among plants, care for them, and learn what they have to teach you, you would absolutely, positively know that they feel pain and joy, and absolutely can suffer. This doesn't excuse causing animals to suffer when they are raised for food. All living beings (plants included) that are being raised for food deserve to be raised in healthy and natural conditions where they can flourish and be happy.
Number two, that eating meat or animal products necessitates the suffering of animals. To some extent, simply being alive can lead to suffering - even healthy people and animals occasionally fall ill, for instance. People who care about the welfare of animals would most likely see us all make far more progress if the argument was about how animals are raised and slaughtered, rather than an unrealistic premise that we should all quit doing something that is 100%, perfectly natural to do - consume animal products for our nutrition. Let's get everyone to see farm animals as the wonderful creatures they are, and to care about how they live and die. Yes, if most people knew about CAFO conditions, for instance, they would be horrified, but many would still like to eat meat. There are alternatives available and more and more farmers are making the switch every year.
Number three, that consuming animal products is bad for the environment. The only thing related to this consumption that is bad for the environment is bad farming, most often aided and abetted by bad agricultural policy. The fact is that any animal, properly reared and placed in the right environment, can fit in, be benign, and in many cases actually improve the balance if its local ecosystem. I know a farmer who raises hogs how they are supposed to be raised - on ample green pasture abutting woodlands that they are free to roam. The sows are allowed to suckle the piglets for a full six months and only gradually weaned. The pasture and woods are happy too, populated with birds, squirrels, deer, chipmunks, moles, coyotes, and everything else you'd expect to find in a healthy Midwestern forest. The key is not overcrowding. The animals are free, happy, and healthy. It is extremely rare that this farmer has resort to drugs, but he also believes, as I do, that a sick animal should be treated just the same as you would treat your own child when ill. Fortunately, he doesn't have to deal with illness very often in his very healthy herd.
Pastured animals' wastes are very effectively returned directly to the soil, fertilizing what they have taken. Properly managed pastures can actually improve marginal land, eventually making it fully productive by controlling erosion, improving organic matter and water retention in the soil. Obviously, a poorly managed pasture can have the opposite effect, which is one reason we may all be better off changing this debate into how to better rear farm animals than insisting on a dishonest set of choices.
It is possible for a responsible, thoughtful consumer of animal products to have as neutral an impact on the environment as a vegan, especially if the vegan regularly consumes imported or non-regionally produced foods, foods produced in monoculture, GMO's, and foods from producers with poor land management practices. A cow, a few pigs and chickens, and a four-acre garden, and my grandparents raised thirteen kids and never made more trash than one standard kitchen can per week. In contrast, I have personally known vegans who fill and entire kitchen can themselves every couple of days.
True, we can't all raise our own animals and gardens these days, but we should all do what we can, and farmer's markets now make it possible to eat much as my parent's family did - simply, and benignly, from your local land, from farmers who care about the same things you do - healthy and happy animals, healthy soil, clean water, and good food.
Number 4, that eating animal products is unhealthy. Here's an example - this hog farmer can't find a market for all of his fat trim that traditionally would have been sold into a vigorous market for lard. Instead, nutritionists (largely funded by huge food and pharmaceutical interests these days)would have us use vegetable oils - heavily processed and grown in monocultures - and very bad for the environment (but great for ADM's bottom line.) Most vegetables oil are also nutritiously empty calories, providing almost exclusively omega 6 fats, leaving your body hungry for 3's and 9's that are found in much greater abundance in animal fats raised on pasture.
Our family, raised on lard as the main cooking fat, never had weight problems, but we didn't eat much either - what we did eat was very satisfying. Butter, for another instance, is rich in A, E, Choline, Phosphatides, and Beta Carotene. Eat some butter and your body will know it. Eat some processed corn oil and you are just as hungry as you were before, only fatter. Good fresh lard is also much more useful to your body than vegetable oil, and it is being wasted while we import monoculture-grown genetically modified palm oil from butchered Indonesian rainforest.
Yes, animal products can be unhealthy if you eat too many of them, or the wrong kind, but the better solution here for the health of the planet and our bodies may be paying more for higher quality animal products and eating less of them. Pastured animals rely on less grain feed, which is not good for their health either. Wild plants provide a much better balance of fatty acids, proteins and minerals, and that contributes to a similar quality in their meat, dairy, and eggs.
In many cases due to digestive issues and allergies, it can be much more productive for someone to eat a small quantity of animal protein than to try to cobble together adequate protein from seeds, nuts, or beans, all of which cause problems for some people.
As part of a diet that includes lots of fresh vegetables and fruit, my consumption of animal protein (nota lot, but some nearly every day) has me feeling much, much better (also weighing less with better cholesterol and triglyceride numbers) than I ever had as a vegetarian or a vegan. I also am, and was, a nutrition freak, and knew what to do to get my protein, vitamins, etc. It just didn't work for me. If it works for you - great! But I, having lived that way myself for years, will not allow my morals or intelligence to be questioned or insulted. I make tough choices every day to be better to animals, plants, and the environment. To just "go veggie" but continue consuming CAFO dairy or monoculture grain, fat, vegetables, etc. simply shifts the negative impacts elsewhere.
Number five, going vegan or vegetarian is the only moral choice - I've touched on this already, and while it's seldom portrayed as the "only moral choice," that is quickly the conclusion you are forced to draw when you engage such narrow argument. The problem is, as I have concluded after years as a vegetarian and one as a vegan, and now again as a meat eater, that the issues are far more complex than the typical veggie advocate will allow. Blessed are the veggies who see the broader landscape!
I greatly admire vegans who can make the choice they do and negotiate the many environmental and moral hazards of consuming plant products, and stay optimally healthy all of the time. I count a few among my dearest friends. It is definitely easier to pull off in a major city with vibrant farmers' markets and ample opportunities to eat locally and organically. The ones I count among my friends also are the type to educate people about the broader issues in our food supply, rather than insist that their way is the only way. Going consciously vegan is a choice, however, and they will let you know that it's available. Then it's up to you. Let's be realistic though - it's not a realistic choice for many, probably most, people.
Ovo-lacto-vegetarianism is something I struggle with. That was my diet for eight years, until I woke up one day and realized that I was eating the eggs of a hen who would one day no longer be able to lay. What would happen to her? What about the cow giving my dairy? Do you bury them somewhere? Throw them in the garbage? Send them to a rendering plant to be fed back to other animals? Used as fertilizer? None of them good or practical choices, I realized I should just stop eating dairy also. But, I wasn't healthy as a vegan, in spite of extensive nutrition study, and meeting all of my protein/vitamin/mineral requirements.
Still, since that day, I can't understand the logic behind an ovo-lacto vegetarian diet, which is why sactimonious ovo-lacto-vegetarians drive me crazy. Certainly, they have the right to that choice, just don't tell me it's better than mine and use animal welfare or the environment as your club.
My pastured pigs and dairy are far more happy than your CAFO eggs (yes, most "free-range" eggs are from miserable hens) and tortured monoculture crops!" See, even a meat-eater can push a false choice! Of course you can be ovo-lacto vegetarian and be a friend of the animals and plants if you make the correct choices. But, if we all want to see justice for farm animals and the soil and lakes and rivers, let's be honest about what's at stake and where the problem lies: bad ag policy, bad agribusiness corporations, bad farmers, and careless tradespeople.
We all have choices, but the moral hazard isn't in eating meat - it's perfectly natural and acceptable if the right choices are made. I also believe it is morally reprehensible if the wrong choices are made. The moral hazard is in consuming the product of any living being, animal, plant, fungus,or otherwise, without seeing to the good care and grace of their lives, and humanity in their slaughter. It also means respecting what they have given you by making sure their gift is fully used and not wasted. That means if you want to eat a pork chop you should be willing to use lard, or if you want to eat filet mignon you should be eager to indulge in tacos de lenguas just as often. Enjoy the delicious, rich flavor of broccoli florets? Find a way to eat the stems or be sure they are composted (a risky proposition with a brassica - perhaps better to eat them.)
Since I've gone from omnivore while growing up to vegetarian to vegan and back, it's been maddening to see the intellectual dishonesty both sides employ when arguing about the best way to eat for ourselves and the planet. I hope we can get beyond that and get to enjoying plants and animals for the wonderful gifts that they are. And, we give farm animals and plants a gift too: life, by selecting them, breeding them, raising them. Let's see to it that they all have lives worth living.