I used to be a "don't have kids" type of person, but my primal needs to have children have caught up with me and now I have a different perspective.
If I want someone to talk about how they grew up on a permaculture-ish farm where they learned how to live sustainably, I have to raise one. If I raise one, then that person will be able to talk to people about the way they were raised (I'm ready for them to not like it, but that is out of my control). That person, whether they like it or not, will use some principles that I instill in them for the rest of their life. Some of those will be sustainable.
If you don't have a baby, we have one less person who can carry on the things you know, believe, and do. You lose the bad, sure, but you lose the good too.