"Noone should be vegetarian"
I think this thesis is hardly defendable. There are no obvious downsides about veganism and vegetarianism for society, animals, or the environment. Fake arguments often brought up include:
- "You're eating soy, thus destroying rain forests." - Yes, I'm eating soy, no I'm not destroying rain forests with my consumption (not at all, or in some cases at least not in the amount a meat eater is).
- "Plants suffer, too." - Your meat animals eat more plants than I do.
- "You're a strain on the health system." - Obvious nonsense.
- "You're giving up an important part of your cultural inheritance by not eating this traditional meat dish." - Let's just write down the recipe, take a photo of it, put it in a book and call it history.
But since your question is not very specific on the way how vegetarianism is opposed, let me add some other readings.
"I shouldn't/won't/can't be vegetarian"
Individual health reasons are often brought up. They might be a reason. That's a separate question.That's a separate question.
Further fake arguments include:
- "I'm an athlete and a vegan diet will decrease my performance" - There are many successful vegan athletes.
- "I will be ridiculed upon by my friends." - While this is, in some places, a valid argument, it speaks strongly against that person's friend, and they are best advised to search for new friends elsewhere, who would accept such a change.
Of course, people need time until they can overcome arguments like these, but on the long run, none of them should be an actual problem (except for health reasons, possibly).
"I'm indifferent towards vegetarianism"
That's the toughest, and the only one that can't be debunked. Someone might just have different ethics. Some people think that it is ethically justified to kill an animal for food. I don't see a way to argue against that on a rationalist basis.
The only counterargument I can imagine goes by convincing that person that they actually do feel compassion with animals and that the modern meat industry has removed the animal suffering so far from daily life that the mismatch between compassion and meat eating wasn't perceived. Then that person might, in the long run, adapt different ethics that support vegetarianism.