Well liberation came through technology and politics where women who were quite dependent on men became no longer dependent on men economically or in fear of being impregnated by them. You could not escape that reality before, and so single families proliferated alongside a legal system which encouraged that rather than the patriarchy which forced people to stay together for the kids even if they could no longer stand each other -- or one party was abusive towards the other.

An alternative theory, and I think much larger reason for the destruction of the family unit is that individualism was heavily marketed by mainstream media through the 20th century. People shifted their wants and desires in marriage from one of creating a family unit to that of finding personal enlightenment.

The excellent documentary The Century of the Self covers the evolution of marketing, the baby boomers and the psychology behind individualism:

https://en.wikipedia.org/wiki/The_Century_of_the_Self