We can believe something, and believe that this belief influences our actions, but that doesn't mean that it's true. For example, I don't heat my appartment. I could say that it's for the planet. But the reality is that it's not for the planet, it's that I don't need to heat my appartment for it to be at a confortable temperature for me. The consequences are still good for the planet, but this is a consequence, not a reason.

If you think children are a massive pain and your rationalization for that is that it's for the planet, there's nothing wrong with that. That may be also a thing people say so they won't get asked too much questions. I've heard that people can put a lot of pressure on other people to make children, the planet argument might be a good justification/counter to avoid being constantly questioned.

> We are capable of believing something without willing to go to the ends of the earth to be consistent with that belief.

I think my point here is that there's a notion of "intensity" or "importence". Having children is a strong belief in many people, and to counter it, you often seem to need a strong belief of the type "climate change is very important", instead of a less strong believe in the type of "I just don't want to".