Nobody here is arguing that our actions aren't causing harm.
If you know, personally, as a single individual, that your actions cause harm, why do you continue to do them? Do you have other choices you could make?
The argument is that a political movement aimed at asking people to make individual changes a) alienates anyone who is unable to make those changes and b) doesn't seem to be a form of activism that solves problems.
The problem is there is too much green house gas emissions. It's a pretty clear problem.
There is a simple way to help, you can actually do, that doesn't require anyone other then you to perform. Instead of emitting say 10 pounds of CO2 a meal, you only emit 5. You could convince someone else to follow your example, and you'd cut their emissions.
The atmosphere doesn't care how the CO2 gets there. Once it's there, it stays for thousands of years. Anything you can do, no matter how small, to reduce your footprint, helps. Start a political organization, I'm not going to stop you, but when I see you unwrap a burger, eat a steak, I'm going to ask why you needed animal protein instead of plant protein.
The oceans are dying. They are being over fished. Something simple you could do is eat less fish. The fish you don't eat will stay in the ocean. The solution to the oceans dying isn't to keep eating fish.
Looking at it another way... Are you trying to say you are somehow justified in emitting this extra carbon because others are? Or because your contribution of those 5 pounds doesn't matter? Sure. Go ahead, emit it. Many of of ... won't. In fact millions of us won't, those of us trying to solve the problem. Peak dairy and peak meat eating in the US has already happened. Individuals are choosing to cause less harm. I'm not the only person doing this /r/vegan is a place. This isn't new information. Environmentalists are starting to push, hard, for vegetarianism.
I'm going to tell you a sad fact, and you can run screaming from it. If you throw out a single piece of litter, it probably won't matter, but if everyone does it, suddenly there is garbage everywhere. If there is garbage everywhere, and everyone around you is doing it ... does your garbage smell any better? Hypothetically, if you wanted there to be less garbage, do you think the solution would be ... you continuing to litter since its pointless to stop?
Are you waiting for a law to pass? Extra taxes on meat? Government rationing of fish or Beef? Food carbon credits? National recognition?
If you know you can emit less, but choose not to, what possible justification can you give for anyone to emit less, when you won't, by choice? If you know the oceans are dying, what possible justification can you give for eating fish? "Everyone is doing it, it doesn't matter?" That's the best you can do?
Fun story
There is this guy walking along the beach, after a storm, and he's throwing starfish back to the ocean.
Someone comes up to him, and goes, "What are you doing?"
"Throwing the starfish back, so they don't die".
"There are too many, it's a waste of time, it doesn't matter."
"It mattered to that one."
Your example could be an example to others. You could explain to them, their choices do have consequences, and they (like you) can run all they want from this knowledge, but at the end of the day, you still have your share of responsibility.