Very good point.
However, I think that in my society, first-world American society, if you don't have a certain amount of money, you can't generally have a family even if you want one. This is partly because you're less likely to have a long-term spouse if your income is low, and less likely to persuade your spouse, yourself, and your friends and family that having a child is not irresponsible given your low income.
It's screwed up that in the modern world that's how it works, because as you rightly point out, it has not long been this way and is still not this way in most places. However it's the life I was born into, and that's okay. I'm not bitter. It's certainly not an unwillingness, rather a prediction. I am personally willing to raise a family in poor circumstances, but perhaps only one or two children at the maximum.
And even then if I can barely feed myself I will not have children. In the United States, doing such a thing would be really irresponsible and ultimately be a burden to others.
So to answer your question it's not unwillingness, it's the economy and culture I live in, coupled with my life choices. In some sense I am unwilling in that I am not going to change my lifestyle in order to be more likely to make enough money to have a family, you are right in that. I am not however unwilling in the sense that I can still have a family in poverty and just won't. I don't believe that if I am poor that I can, or can responsibly at any rate. Know what I mean?