I find it bizarre when my sister badgers me all the time to consider having kids when she appears to resent motherhood massively.
There's your answer, OP, or at least some of it -- she doesn't see why you shouldn't have to traipse through the gruntwork of parenthood, and 'get off scot-free' from nappies and homework and school holiday prices. The idea that you have freely chosen the 'easier option' (as she clearly sees it) and are at ease with it is very unnerving to some conformist people, who need to be surrounded by people who've made the same decisions.
To people who never thought about whether or not to have children and just drifted into it because 'everyone does, don't they?', watching someone make a different decision flags up the possibility that they could also have chosen not to have children, and it freaks them out. You are forcing them to see their own decisions as what they are, choices, not some kind of inevitable life stage like death.
I was happily child-free until 40, and despite having decided to have a child, I am entirely clear on the fact that my life would just have been differently good, and equally valid, had I not had DS.
I am surrounded by child-free women, including both my sisters, and do not feel a flicker of judgement or 'concern'.
Having a child because someone else tells you you might regret it later on when it's too late is a mad reason to have a child. And I agree with whoever said up the thread that giving birth to your own geriatic care is equally odd as a rationale.