Vegetarianism is the purposeful exclusion of meat from the diet. I have never been a vegetarian, and, having grown up in a meat-eating family and having developed a love of meat, I probably never will. However, despite not being a vegetarian, I have to admit that vegetarianism is morally superior for the majority of real-world ethical systems. Here is my argument:
P1: It's immoral to purposely injure something that can feel pain without a sufficient reason. P2: Killing animals injures them. P3: Injuring an animal for food is rarely a sufficient reason. C1: Therefore, there usually immoral to kill animals for food.
To elaborate on my argument:
P1: It's immoral to purposely injure something that can feel pain without a sufficient reason.
This is an accurate statement in every ethical system I've encountered. Allowances are made for particular cases, but, in general, they have prohibitions on injuring innocent people. People understand that pain is unwanted, and purposely causing it to others for no good reason, is immoral. Possible reasons are discussed in P3.
P2: Killing animals injures them.
Some may argue that killing can be conducted in such a way that it won't cause physical discomfort, which is true, however, we wouldn't accept such an argument regarding people because we understand that people have future plans and desires that are cut short when killed. Likewise, by killing someone, even painlessly, a great deal of suffering is caused to the loved ones of the deceased. It can be argued that animals don't desire to remain living, or that their fellow animals don't care if they die, but anyone who has been around animals for awhile knows that this is not true.
P3: Injuring an animal for food is rarely a sufficient reason.
People are rarely in the position of needing to kill an animal to prevent starvation, and, if they are, this is usually due to poor planing. However, I will accept that killing a lesser animal to stave off starvation is not immoral.
Regarding sustenance, millions of vegetarians have shown that humans can, fairly easily, thrive on a diet devoid of meat.