"Natural" reasoning for Western people Edit
Most people living in the West are raised on meat. Under these conditions, moral claims along the lines of "eating meat is a murder" sound quite intimidating. A person is likely to search for ways to reject such a claim, since accepting it is equivalent to consider oneself a mass murderer. Moreover, if one feels obliged to assume such a strong position and "converts to vegetarianism", this may complicate relationships with other people, who remained "mass murderers".
At the bottom line, it seems sound to assume that reasoning based on "equivalence" of killing an animal (a norm in contemporary Western culture) and a human (the worst possible crime in a secular society) actually makes a lot of people avoid vegetarianism. This reasoning is also somewhat unnatural, since most of us feel that there is a certain "hierarchy" of living beings. We feel that killing an insect, a cat and a human are three different things.
So, assuming that killing an animal is not a murder, why would one become a vegetarian? There are numerous reasons.
Killing an animal Edit
"Would you kill an animal?" Most of us know people who would answer "yes". Nature or nurture - you name it, but the fact is that people are different, and some are closer to "carnivores" than others. For these people, it is natural to eat meat, and trying to persuade them to act otherwise is probably a waste of time. In particular, such a person is unlikely to reach as far as this line in a text advocating vegetarianism.
On the other hand, someone who is reading this will probably have hard time killing an animal. Skill aside, many people will only attempt to kill a mammal if they are absolutely forced to do so. Others are too "sensitive" to enjoy fishing, and may feel uneasily watching a fish killed.
For these people, and there are quite many of them, eating meat is somewhat unnatural. Normally, we'd only hire someone's services for skill, but buying meat means hiring someone for doing a dirty job for these people - something they wouldn't do in any other situation.
Eating a dead body Edit
Sometimes meat doesn't look like a dead body (as in hamburgers), and sometimes it does (as in chicken or fish). Anyway, the "sensitive" among us usually have quite strong imagination. It is easily fuelled by the sheer knowledge that we are eating a dead body, and is helped by details such as bones or eyes.
This makes the meal somewhat less enjoyable. We eat meat, since it's a very easy way to satisfy hunger, but we don't like it so much.
Killing for luxury Edit
If one has to kill animals to stay alive or even to stay healthy life, it's perfectly legitimate. In the Third World many poor people have to make use of whatever reasonable protein source they can get to keep themselves and their children from protein defficiency.
But killing animals when it's not necessary, for fun, is usually frowned upon in our society, to say the least. If kids are trying to hurt a cat, most of us (including a lot of the above-mentioned "natural carnivores") would try to stop them. If an adult kills cats for fun, we'd rather have him in prison or hospitalized.
But is there any other reason for eating meat except "fun"? The taste and the accessibility fall into this category.
The industrial approach Edit
The way we get our meat is more cruel than, say, the killing of a cat by throwing stones at it. The meat we eat is the result of mass production, with production costs systematically minimized to raise profits. The animals, born to be slaughtered (the cat has better chances), are raised in quite bad conditions. The "sensitive" among us will suffer quite intensively if they observe the conditions on factory farms.
This is another reason why eating meat the way we do today is unnatural. An animal caught by a hunter or raised by a farmer at least has a life before it's death.
This is also a reason to consider going further than "plain" vegetarianism, since eggs and milk, produced without actual killing, are also subject to mass production. And the process is not pretty.
Meat is not the healthiest food. This reason is listed last since the younger people rarely mind health, and the older people rarely change habits, so this most practical of all reasons is also the least likely to be persuasive. Still, the reasons for meat being unhealthy are worth mentioning.
First, meat by itself can clutter your veins. Second, the meat lived and died in fear and pain, which may affect the chemistry. And third, we might feel uncomfortable when eating meat because of reasons mentioned above, also affecting the chemistry.
None of these things are "purely scientific" - you need a very large body of carefully collected data to reach a truly "scientific" conclusion about a complicated subject. But less solid evidence would probably be enough for us when it comes to our health - for those of us who are aware of health issues, to be precise.
While killing an animal and a human is far from "the same", most of us feel that this shouldn't be done without necessity, and even then we'd prefer to do it in the less painful way. For secular people, the basis for this is identical to the basis for the prohibition of murder, namely, those of us who can identify with other beings do not want to cause unnecessary pain.
Eating meat is not murder, but it is an unnatural act for a lot of people. These people will probably feel better as vegetarians.