Wow, this was a long read. I just have to add my two cents. There were too many posts to get all the quotes, (sorry) so I tried to represent the ideas here fairly.
Argument #1:
Humans have been eating meat for a long time so there's nothing wrong with eating meat now.
Humans have been doing a lot of things for a long time, like killing, writing poetry, worshiping ancient gods, exploring, enslaving and having children. Things are good or bad because of what they are, not how long people have been doing them.
Argument #2:
It's natural to eat meat.
"Natural" is a loose term based more on a person's feelings than facts and gets applied to almost everything. Usually people use it to mean "it should be done" or "it shouldn't be interfered with," but that alone isn't a good enough argument. If you mean "we evolved to eat meat so we should" then say so and we'll discuss biological determinism. If you mean "meat is god-given" then we'll discuss theology (but not in this thread).
Argument #3:
Animals eat other animals. Humans should be able to also.
From an ethical/moral standpoint I'd say animals don't know any better. They only have instincts. Humans have thoughts, morals and ethics. Humans have the capacity to know better.
Argument #4:
Humans need to eat meat to survive/stay healthy, particularly people who live where farming isn't possible.
People in developed countries have access to all kinds of foods and nutritional information to know that they can stay healthy and provide themselves with all the protein, vitamin and whatnot from non-animal food sources. If you are one of those guys in Africa then go ahead and eat your meat since you probably can't grow soybeans.
Argument #5:
It's my life and my choice.
Your choice to eat meat affects other people. For instance: for every cow that's bred for meat an exponential number of people could be fed on plant foods (grains, fruits, vegetables) with the same resources (land, water, etc.) used to raise that cow. There's also the issue of "cow farts" (methane gas) released into the atmosphere and compounding the problem of global warming.
Also, I didn't see any response to this (unless I missed it; correct me if I'm wrong).
Every species is parasitic. Water can be cleaned, and grass can grow back. We are using renewable resources.
Thankfully, most people are not anything like Americans.
Renewable resources can still be exhausted if they are not given time to renew.
Cleaning water is expensive and not everyone in the world has the means do it. Some water is too polluted for our technology to clean or our technology takes an inordinate amount of time to clean it.
Grass will not grow back (or will grow back at a very slow rate) if its environment is damaged too greatly or was nutrient-poor to begin with. Overgrazing, drought (often a man-made side effect of overgrazing which renders soil exposed to the sun where it previously had plants covering it), erosion (a side effect of overgrazing and drought which leaks nutrients from the soil), pollution or salinization of soil all impede healthy plant growth.
More and more people are aspiring to become like Americans. China (home to over one billion people) is rapidly growing closer to First World levels or per capita resource use. That's just bad news for everyone because it means competition for dwindling resources.
So, raising animals for food is damaging to the environment and damage to the environment is damage to humans.
/soapbox
