We've destroyed a lot of natural habitats, and we know that these habitats contain a great deal of biodiversity. We just don't know what would have happened if we hadn't destroyed those habitats. My guess is that we wouldn't be facing a mass extinction event, but there's no way to check.
Even if there was only a 10% chance humans were responsible, I would still rather play it safe than ignore the issue.
And I certainly take issue with your vegetarian example. What other people eat is none of your business. It sounds almost funny, but that would be food fascism--is there any other word for it? Do you really want some bureaucrat sitting behind a desk writing memos telling you what you are and are not allowed to eat? How can you think that's a good idea? The more governments try to control people, the more people are harmed. Doesn't history make this obvious? Why do people seem to think, "Yeah, but we can get it right This Time"?
edit: Never mind, I read your answer up above. Well, anyway, a judgment call.
It sounds like a really interesting idea for a computer game: you're the president of the world, and an asteroid may or may not be going to hit the planet in so many years. Can you bring the planet's resources to bear in time to prevent it? Can you develop the technology and send the mission without bankrupting society and causing mass unrest? Can you delay that preparation and develop better technology to figure out if the asteroid will really hit?
Or do you play the villain, letting the asteroid hit, but building fallout shelters to survive with your friends while everyone else perishes? Can you do that without the populace catching on? Can you get it done in time? Can you outlast the "asteroid winter"?