I didn't say anything about how anyone should have fewer kids. However, intelligence and political attitudes are highly heritable, so when smart educated middle class people (regardless of race or country) decide not to have children so that they can do their small part to "save the planet", what they're actually doing is their own small part in helping ensure that the next generation is turned over to people who are less intelligent and don't share their political beliefs.
1) Because you're treating an idealized version of "the environment" as a terminal goal.
2) The people most likely to agree with your argument are precisely the ones who should be having more kids.