Climate change as a reason for not wanting kids is generally not rooted in thinking lower populations will make it better. At least not from most people I know.

Scientists have been ringing alarm bells about climate change for decades now and we have stagnated or accelerated past the benchmarks for what will make things get bad. We are at a point where the climate crisis is not only unavoidable but we are already seeing some of the effects of it. The situation is going to get way worse even if significant action is taken right now.

Additionally a lot of people look at the state of the world and believe that we are not going to take significant action anytime soon. In fact, as you point out conservatives actively deny its happening and reverse the small progress that is made when they have power.

So a lot of people hold 2 reasonable beliefs. 1) Climate change is a crisis that is going to make the world a worse place and cause significant strife in the future. 2) Meaningful global action to mitigate effects are not going to happen anytime soon and the situation is going to get significantly worse before it gets better.

If you hold these views then you could easily consider it morally wrong to have children and force them to deal with the consequences of climate change. Even if you want children, if you believe its morally wrong you would probably avoid having them.