Also, childless people, on average, will cost more to society than they contribute. To be conservative, pension programs (including social security) rely on there being at least as many people around in the future to pay the bills. If you don't have any kids, someone else's kids are paying the bills.
In a vacuum, if a couple has only one child, that child had better grow up to support those two people in retirement. If four grandparents only have one grandchild, it's even worse.
The current answers to that problem are immigration (in which case someone else is still paying for benefits) or incredibly low take-home pay for workers (lots of withholding for personal or public retirement funds).
There are enough people on earth, and the global population is increasing. You don't need to personally replace yourself; immigration can handle that task. It's only when you don't allow much immigration that you have to start getting worried (such as with Japan)
On the other hand, the most significant way you can reduce your environmental impact (short of ending your own life) is to not have children. In that regard, it's the most responsible decision anyone can make.
In a vacuum, if a couple has only one child, that child had better grow up to support those two people in retirement. If four grandparents only have one grandchild, it's even worse.
The current answers to that problem are immigration (in which case someone else is still paying for benefits) or incredibly low take-home pay for workers (lots of withholding for personal or public retirement funds).