“if the animals weren’t bred to be eaten, they wouldn’t have been born at all”.
Unfortunately, the statement is accurate. I thought about this a long time before going vegan, before ultimately figuring out that although the statement is accurate, it is not a successful defence of meat eating from a moral perspective.
And there are three separate lines of argument that explain why from different angles.
The key to why the argument seems to work but doesn't is the way it conflates two separate issues - the breeding the the killing/eating - as if they have to be related. But instead they should be looked at separately.
1. Is it right to breed the animals in the first place? Answer: maybe, probably no given the way it's done and their breeding is manipulated to our needs at the expense of the animals. Also, why do we have the right to control who lives and for what purpose?
2. Is it right to kill them? Answer: no. The fact that an animal has been bred for a purpose does not justify the moment of putting a knife into its throat or whatever.
BENEFITS OF VEGANISM
Now it may be true (although this is debatable and most vegans would usually not accept it) that eating meat is good for farm animals, since it's possibly better for them to have a crappy life than none at all. But, that does not make if OK to kill them. Eating meat is wrong from a moral perspective, even though the farm animals don't benefit from veganism.
However looking at veganism only from the point of farm animals misses the point that the less farm animals we have, the less land we need to grow food, and therefore the more land we have for humans and wild animals. As people go vegan, the planet can support its existing humans and wild animals with a better quality of life due to more land per person/animal, or allow a greater population of humans and wild animals. Humans and wild animals are the true beneficiaries of veganism
(note that even most vegans and animal rights activitists do not accept this, or even realize it, and disingenously promote veganism as an animal saving diet).
However, using examples comparing with humans can be instructive also.
Unfortunately it logically follows from your Mom's statement that I could breed humans for food and use the same argument. Imagine I had a factory where I genetically breed humans from a test tube, and due to a defect in the genetic process their intellectual development is stunted and they cannot ever become more intelligent than a 1-year old child. This means that they cannot be ever be as intelligent as a pig. If you propose shutting down my factory, I can make the same argument as your mother. “If the humans weren’t bred to be eaten, they wouldn’t have been born at all”. And yet most of us would instinctively know that breeding humans for food would be wrong, and would not change our minds if the humans were less intelligent than animals as I show above.
Let's say that there was another species of animal on the planet more intelligent and powerful than humans, and that they bred humans for food and ate them themselves. By using factory farming techniques, such as keeping humans in cages and giving them crappy food, they are able to increase the human population to an unnatural level of 100 billion given the earth's resources so that they can constantly eat humans. Now the same argument applies: “If the humans weren’t bred to be eaten, they wouldn’t have been born at all”. And yet most of us would still be morally repulsed by the idea and not OK with it even though more humans would have the opportunity to live.
It logically follows from your Mom's statement that two adults could deliberately have sex to produce a child in order to eat it later, fatten up their own human child, and eat it, and once again, same defence. “If the child wasn't bred to be eaten, we would never have had the child in the first place.” Surely most of us can see that the fact the child was only born in order for food - and may even have benefitted as a result - does not make it OK to kill the child, and does not make the whole process OK.
It's only our speciesism - ie our species bias - our total lack of consideration for other animals, that would permit us to use arguments in favour of eating animals that would, if applied to humans, but not just morally wrong but blatantly absurd. If the argument completely fails when applied to humans, how can it be completely OK for other species?
I will write a second post to explain how to actually reply to the comment when it comes up.