Since "Toen was geluk heel gewoon" is based on The Honeymooners (1955), the Kooiman and Stokvis households were both portrayed as childless. While the original never gave an explanation for this, an explanation for the Dutch version is finally given in this episode: Cox and Pleijsier later admitted that they often wondered why the couples didn't have any children despite living in the 1960's. The Kooimans are portrayed as being childless due to Jaap hating children, much to Nel's dismay. Simon and Zus did want to have children, but never managed to.
Zus and Simon Stokvis never managed to have children due to Simon's semen being too slow. This was an in-joke, referencing Sjoerd Pleijsier's real-life trouble with having slow semen. In the end, Pleijsier got children anyway.