If you're having children you can't afford, you are irresponsible. That is simple and easy for anyone to understand. If you then take your irresponsiblity and pay for it with the tax money of responsible people, you're rewarding the irresponsible people and punishing the responsible ones.
The part where I didn't have children I couldn't afford is what makes it particularly galling. I gave up something and they didn't, but I'm punished and they are rewarded.