It isn't. It never has been....necessary for everyone to reproduce. I'm the last guy who'd instruct everyone to have children, especially people who don't want children. There are already too many people making babies they didn't want.
It's just a philosophical argument, not a mandate for EVERYONE to rush out and start a family, to save the human race from extinction. I don't believe there's ever been a generation in which every man fathered children(maybe, Adam's generation?).
At root, it's just a reaction against all the "this is the LAST human generation" hysteria. Every generation since WW2 has had its pending apocalypse. In the Cold War, we were all anticipating nuking each other into oblivion. Today, three out of four millennials actually believe that humans exhale a poisonous gas that's going to cause the Earth to explode in a giant ball of fire, in 12 years, unless we all become Yurt-dwelling bicycle messengers, immediately.
I completely respect people who don't have children, because they don't want children, and those who don't recklessly run around fathering children they don't want. However, when anyone utters banal leftist talking points, about how supposedly irresponsible it is to bring children into the world in such a uniquely perilous era as this(as if they'd ever have an opportunity to perpetuate their DNA, in some safer era), it triggers my gag reflex, and a torrent of 2 cent pieces erupt from me.