If we didn't raise animals for food, then they
would never have had the chance to be born and experience life at
all.
Considering how the vast majority of farm animals are currently raised in modern industrialized agriculture, these animals would have been far better off never having been born. The best moment in these animals lives is when they finally die, because only then does their suffering finally end.
However, if the animals are raised and slaughtered humanely, some would say that it is better for the animals to have experienced life for a brief time before slaughter, rather than never have been born at all. My response to this is the following.
Once an individual is born, we have the same obligation to act ethically toward them as we do towards everyone else. This is not changed by the fact that the individual would have never been born in the first place without our intervention. This is the reason that child abuse is immoral. Even though the child would have never existed without his parents, this does not give his parents the right to physically abuse him.
If we hypothetically lived in a cannibalistic society which bred and raised a race of humans for food, then we would be correct in condemning this practice, even though without it, these humans would have never existed in the first place.
In spite of all this reasoning, some people insist that by refraining from eating meat, I am guilty of depriving farm animals the chance to be born and experience life. One way to respond to this is to point out the inherent inefficiency of using farm land to grow crops for livestock, instead of growing crops consumed directly by humans. If everyone ate a vegetarian diet, then a much larger human population could be sustained with the available resources.
Therefore, if I am "guilty" of depriving farm animals the chance to be born, then people who eat meat are "guilty" of depriving human beings the chance to be born and experience life.
Follow up questions:
How are farm animals currently treated?
Isn't it unfair and insulting to compare the murder of a human being with the slaughter of animals?
How common is cannibalism in the animal kingdom?