I think the nicest mean thing I have been called is “selfish.”
When I tell people that I don’t want to have children, they usually look at me with disapproval before asking, “Why?” — with more disbelief than curiosity. Sometimes, if I’m really lucky, they will tell me how and why all childfree people are wrong. They won’t refer to me specifically, as in why I (Isabel) am wrong. No, instead they will generalize. It’s more polite that way.