I am a vegetarian and I have been for almost 20 years. I mostly eat vegan, my two indulgences and exceptions are occassionally some raw milk cheese and very rarely some dark chocolate ice cream. I eat vegetarian for a lot of reasons: health reasons, ethical reasons, environmental reasons; and, really, I just find meat to be gross. Who wants to eat a dead, rotting animal carcass?
Even if meat was grown in a petri dish I would probably not eat it because ethical reasons are not my only reasons. But I have been thinking about the ethical issues a lot lately and have come across some troubling issues.
The main ethical argument against meat, at least the one I make, is about the factory farming system where animals are treated like industrial raw materials. Besides the horrible ways the animals are treated is the fact that the consumer is so far removed from the process that many people don't realize that the meat they get shrink wrapped in plastic at the grocery store comes from an animal. So ethically I have no problems with small farms where the farmers actually interact with the animals and kill them personally, not by feeding them into an assembly line slaughter-house contraption. I do have problems with people buying and eating meat if they have no personal experience with the killing and preparing of animals, but I digress.
My point was about the animals that are raised for food - primarily cows, chicken and pigs. These animals are domesticated - they could not survive in the wild. If humans disappeared they might adapt or they might go extinct, it's really hard to say.
Let's consider a hypothetical situation - everyone in the world goes vegan. This argument also works if everyone goes vegetarian, but not as well. We have no need for food animals anymore. What is going to happen to all of the domestic animals? They likely can't survive in the wild, maybe a few could.
These species have evolved over millenia and adapted to a symbiotic existence with humans. We needed them for food and they needed us to survive as a species.
So these animal species either go extinct or have their populations greatly reduced. The animals that survive in the wild will likely have far harder lives than they would have on the farm. I'd imagine it would be worse to be eaten alive by a bear than killed by a farmer, and it certainly would be harder to find food.