Many people seem to think that single parent families and blended families were a rarity until the increased prevalence of divorce in recent decades. Genealogy tells a different story. In researching my ancestors and those of other people, I have indeed found very few that were divorced, but I have also found very few children who grew up with both parents, and many who grew up in blended families.
People tended to die much younger in previous eras, and the surviving spouse often remarried. Many women died in childbirth or as a result of complications from it. The combination of fewer safety protections and lesser medical knowledge meant people were much more likely to die from accidents and common diseases, even well into the twentieth century. People do tend to be aware of these facts, but they often don’t extrapolate from that knowledge what it meant for family structure.
When I am researching a family, the first thing I do is try to find it on as many censuses as possible. Because misspellings and inaccurate birth data are rampant on census pages, matching a family on, for example, the 1900 census with the same family on the 1910 census can be a challenge, but it can usually be done. It is easier with larger families, because even if a few members are not on both lists, you can still be fairly certain it’s the same family.
It is not at all unusual to find a family headed by a widow or widower, even in the last publicly available US census, from 1940. It is also common to find families in which one or both parents have married at least twice, often having children with each spouse. This means many children grew up with stepsiblings and half-siblings. Many had periods in their lives when they had just one parent, and many grew up with stepparents, some with a succession of them.
I have also frequently found families not living together. A couple may be listed as married but living in separate houses. Sometimes all the children are with one parent, and sometimes some are with each. Sometimes some or all of the children in a family are living with grandparents or aunts and uncles. Poorer families are often split up in multiple houses, with even young children working as domestic servants or farm hands.
From what I have seen in genealogical records, there have always been many single parent and blended families. I have not come across any era in which the vast majority of families consisted of a couple who had only ever been married to each other and just that couple’s children. It might have been the ideal, but it does not seem to have been the reality.