I genuinely can't work out why people are getting so defensive and feel the need to post repeatedly that they don't want children. Nothing in the original post said there was anything wrong with not wanting children, and OP has repeatedly reaffirmed that since. She is just expressing puzzlement that people frame not wanting them as not being able to afford them.
However, while I think OP is right, I do understand why people see it that way. I might say we 'can't afford' a third child, but to lots of people we could, easily. What we really mean is that we don't want a third child enough to outweigh the downsides, including the financial impact, whereas we did want a first and second that much. If I found out I was pregnant tomorrow we'd manage to afford a third child. If we suddenly had loads more money I would still hesitate over having a third, but it would be much more attractive. So, it feels somewhere between 'we can't afford it' and 'we don't want a third' - neither is quite true. I think the same is true for a lot of people weighing up whether to have children at all.
Clearly financially the best decision would always be to have no children. No one is better off with them than without them. Deciding to have them is always an 'irrational' choice, and so it's always going to come down to how much you want them.