It seems to me that a single parent families are increasingly popular in our society, given the way that many older people react to single mothers/fathers and the couple never being married. Is the single parent family a fairly new phenomenon, or has the taboo just decreased and they were found throughout history?
I can only answer the first question, single parent families have increased dramatically since the 1980s as the table from the census indicates. This is not a special US phenomenon, it is common to the developed world as a whole. I don't know the particular US causes but the decline of stigma is very important elsewhere. This has driven changes in welfare systems and women's employment that help single mothers in particular to manage on their own.
Hopefully someone with more US expertise can add more.
https://www.census.gov/compendia/statab/2012/tables/12s1337.pdf