The principle of contract is a leaking abstraction.
Society has always worked on the principle of obligation. Parents are obligated to protect and provide for their children. Rulers in power are obligated to protect their subjects from external, physical threats. Those well off, while part of a society, are obligated to provide for those who have been hit by hard times.
This is true now, and it was also true before the idea of contract. It may be legal to have lots of kids and give them all up for adoption, but you will be ostracised by your friends and family for doing so. It may be legal for a president to stand down their military and choose not to resist a foreign invasion even when there's a good chance they could defend the country successfully, but the people will stop following the president and there's a high chance of a military coup. It's also happens when the well-off are least happy about having to contribute to society during hard times, they get rounded up and killed.
Not having children is a perfectly valid lifestyle choice, a childless person saying that raising children contributes less to society than they do says a lot about how they fundamentally view themselves.
It reminds me of the polar opposite of what gay man once said to me: "I love straight people, they make all the gay people" in reference to another man talking down about 'breeders'
Their parents chose to make those decisions, nobody forced them. The child did not choose to be born, so why should it bear any responsibility for a decision it had no part in? I can't paint your house then say "hey, I painted your house, pay me now"; our society works on the principle of contract, that I can only make you owe me something for painting your house if you actually agreed to do so beforehand.