The reason alot of people are against it is because they don't understand it.
They are under the impression that humans NEED meat, that it's unhealthy to not eat meat, that if you eat only fruits, veggies and salads, you'll be malnourished. What they don't know is that it's not the meat humans need, it's the protein and iron found in meat. Dogs are animals that need meat, cats are animals that need meat, humans don't. While it doesn't really hurt us to eat it, we can live happy and healthy without it. But dogs and cats, if they don't get it, they are the ones that will become malnourished.
A properly executed vegetarian diet is just as healthy as a person who includes meat in their diet because there are other sources of protein and iron, such as tofu and nuts, as well as fish, eggs and dairy products if you choose to include that in your diet. Kind of like how I have my dogs on a raw meat diet. Most people hear that and think "What? You feed them raw meat? That can't be healthy!". But they too don't understand the diet. If I gave them nothing but chicken legs every day, then yeah, it's horrible for them. But they get chicken, tripe, liver, some fish, raw egg, some veggies, a tiny bit of fruit sometimes, alfalfa....It's extremely good for them.
This is why I'm indifferent, I understand that. For the most part, I really don't eat meat, I just don't like it, I can hardly even stand the smell of a steak, nevermind the thought of eating it. But I can't really call myself vegetarian because I rarely eat vegetables either. Truth be told, I'm a carbivore myself....Give me pasta, grain and potatoes and I'll be happy