The context of the conversation, however, was that people had children as a non intentional byproduct of having sex, which presumably people had more of due to lack of other entertainment options.

To which I thought it was noteworthy to point out that children are effectively no longer a byproduct of just having sex, especially for the demographics in which birthrates are declining. Hence, regardless of the amount of sex people are having, I would expect the birthrate to still decline.