Right, I overstated the implication. More people deciding not to have children does not lead to extinction. It's just that when people say things like "not having kids is good for environment" it comes off as a moral prescription that ought to apply to everyone, and hence those who violate it are the bad guys (selfishly acting to harm the environment). And yet if sustainability -- rather than self-extinction -- is the goal, we need a mix of both types of people.

Am I wrong to interpret it like that?

The ones who insist on having children as an ultimate virtue, I find them to be just as wrong.

Either choice is fine, and the moral objections to either one are unreasonable.

Edit: edited to expand the last sentence.