Modern technology means that only a small percentage of citizens are enough to defend the rest. That's why nobody wants/needs the average J. to fight.
If all of a sudden, say, 50% of the people having children now, in developed countries, would decide to not have children, we'd probably have societal collapse. That's what happened in societies where the population went down by huge percentages in a short time span. Housing markets would collapse, education systems would collapse, healthcare systems would collapse, demand for a million types of things needed by babies and children would collapse (and taking with it many, many professions), etc.
Even if you don't want to have kids, remember:
1. You were once a kid.
2. Your lifestyle is made possible by people that choose to have kids. Especially women, who sacrifice a lot for kids. At least be respectful to them.