He's not saying it's selfish to not have children, he's saying the decision making process relies on determining the usefulness of a given activity, and kids rarely score high there. In the past it worked differently, I suppose at least partly these things were done because of tradition or because of religious encouragement. In the past they weren't doing it in some way that was significantly less selfish than us, but whatever reason they did it for the system would continue unlike it is today