I was mainly just challenging the assertion that having children, with at least part of the motivation being care in old age, is selfish.
I interpreted this as not "I will have children so my children can look after me", but rather "I will have children so there will be younger people around, who can (amongst other things) look after older people and do other important things".
Based on that way of looking at it, I think that not having children but still benefiting from younger people is more selfish than not having children and benefitting from younger people (if either of these is indeed selfish).
Basically, I was questioning why it is more selfish to use a resource while contributing to that resource, rather than not contributing but still using.
Put another way, who is more selfish - a farmer who buys some vegetables, or a software developer who buys some vegetables? Yes, they are both paying for them, so they are not directly exploiting anyone. But if no-one wanted to be a farmer, then there would be no vegetables.
Yes, I know this analogy is a stretch, but hopefully you get what I mean. Anyway, I don't think that either having or not having children is inherently "selfish", but there are almost certainly selfish motivations for each.