Well, answer one to 'why should I pay for X because Y is not my fault' - you might one day end up in such a situation and I am sure you'd appreciate the safety net.
I think it is not OK for you to take such decisions on your ivory tower. So, a thought experiment (called the
Veil of ignorance):
Assuming you are a fully aware, sentiment being about to be 'born' in this Earth. When you are born, you'll be randomly placed in England; a random city, town and between a random couple. You could end up in any family.
You know everything there is to know about the UK. You know that, for example, the 50% of households earn less than £25k and 50% earn more. You know that 99% of households earn less than £80k. You know that x% of households are jobless single parents/end up being single parents, for one reason or another. You know that if you are born in such a unit, odds are that your life is going to be pretty miserable - your life chances will be poor. You'll be less healthy, less educated and will achieve less than most others.
Now, answer the question: should there be child benefit which allows such a family unit to at least sustain themselves at some level?
If you answer no, you're condemning yourself if you fall on the wrong side of the odds. If you answer yes, your chances of living an average life are much higher.
What's your choice going to be?
The good thing about the 'Veil of ignorance' is it is a really nice way to exploit the power and passion behind greed and instinctive individualism for the greater good of building a good social framework. You can apply it to many things; death penalty, whether something should be illegal, taxes and so forth. It's all well and good for you in a certain position of certain upbringing, income etc to dictate terms - but it is another for you to have all that undone and to be randomly dropped back into society, and to
then form these opinions.