Can't or Unwilling

The average Joe says, "I wouldn't want so many children because of the financial burden. I simply can't afford it."

In the words of Martin Luther:

"Although it is very easy to marry a wife, it is very difficult to support her along with the children and the household. Accordingly, no one notices this faith of Jacob. Indeed, many hate fertility in a wife for the sole reason that the offspring must be supported and brought up. For this is what they commonly say: ‘Why should I marry a wife when I am a pauper and a beggar? I would rather bear the burden of poverty alone and not load myself with misery and want.’ But this blame is unjustly fastened on marriage and fruitfulness. Indeed, you are indicting your unbelief by distrusting God’s goodness, and you are bringing greater misery upon yourself by disparaging God’s blessing. For if you had trust in God’s grace and promises, you would undoubtedly be supported. But because you do not hope in the Lord, you will never prosper."

— Martin Luther (Sermons of Martin Luther, The)

With a fresh-off-the-lot, over-sized, gas guzzling SUV/truck parked in the garage, most people who think they can't afford more children are not "paupers" and "beggars". They are simply unwilling.