Having children (and being married) is a tool for survival in harsh environments. Conversely, it can be seen as a trap/burden from a legal and financial perspective in a 'developed' country.
Edit: In the land I come from (south Italy) it was fairly common up to 50s and 60s for a couple to raise many children, as many as 7 or eight.
The reason is dead simple: being the economy largely based on agriculture and being schools not very common, more children meant more helping hands.
That need has basically disappeared.