In fact, people are still expected to provide reasons not to have children, but no reasons are required to have them. It’s assumed that if individuals do not have children it is because they are infertile, too selfish or have just not yet gotten around to it. In any case, they owe their interlocutor an explanation. On the other hand, no one says to the proud parents of a newborn, Why did you choose to have that child? What are your reasons? The choice to procreate is not regarded as needing any thought or justification.