But sex as reproduction was the norm worldwide until the 1920s.
What's different about the Duggars is that of their 19 children all survived past one year of age.
For most of human history, you could expect to lose one child out of two or three before they could get through their first winter, and you could likewise expect to lose one woman out of four to a difficult birth before should bear more than one or two children who lived to adulthood.
I know plenty of families who have eight or more children. And they don't live by all the Duggar family codes and rules.
people have been having recreational sex far before 1920.