How is it not selfish to have children?
Did they ask to be here? Life isn't a bed of roses, even for the luckiest of us. The majority of us like or at least are ok with living, but there are millions and millions of people who wish they weren't, and everybody dies in the end, many of us before our time, and most of us in a state of suffering.
I am perfectly willing to admit I am childfree for self-focused reasons (I don't believe it's selfish, because who am I taking anything away from?). Why can't parents admit that?
And in reality, there are some childfree people who choose not to have kids for completely unselfish reasons -- environmental reasons, really bad genetics, etc. I know a childfree woman who did it for one of those reasons, and she says she actually went through a period of grieving her decision.
I can think of no such equivalent for biological parents.