I still think the "you are selfish if you don't want kids" is a projection of this person's actual feelings about her decision to have children. I have several friends that have children and not one of them have told me this. However, one of my aunts has told me this whenever she gets the chance (of course, we don't talk often) but I could never really get her to tell me what is wrong with not having children that I don't want. Her reasons for me to have kids really come down to "I am suffering for my decision and you should suffer too." But, let me add that I do not think every mother out there is suffering for having children. I come from a family (both maternal and paternal) that has had children before they could afford to do so and they ARE suffering for their decisions. So, needless to say, there is a lot of resentment when it comes to me because I have had a lot more opportunities since I am childfree.
If they think about it, us choosing not to have children will have absolutely NO IMPACT on them, so why do they care? I could certainly care less if they choose to have 1 million children because it is not my problem. To each, his own.