I agree, I don't think not having kids has anything to do with being selfish.

But I guess what they meant to say is just that having kids is hard. A lot of times, people become more selfless and less focused on their own wants and needs after they become parents.

Of course, some people don't actually become more selfless or responsible, and instead they just become bad, narcissistic parents. But the choice to have kids is associated with the choice to be a responsible adult and work for the benefit of others, at least in theory.

That argument makes sense. Though I see it more from choosing to prioritize yourself and own self interests over having children and sharing a life with them as selfish too. I guess we're all selfish one way or another.