It also takes knowing stopping births is possible and having the access to it. In lthe midst of the worst atrocities in human history we have still had sex. Slaves in the American south still had children in conditions that vastly outstrip anything else in modern capitalism. Native Americans still had children as disease killed 90% of their population. As long as there is enough food to keep survivors healthy enough to concieve and carry to term we have always had children. Because we always seek comfort from one another and sometimes that's sexual.

Now we have the opportunity to get that comfort without bringing in the child and we do.