I believe the answer is: Family structures are changing, but families remain important to American life.
The main reason of the changing structures is the increase in divorce rate, and non-monogamous relationship. In the past, almost all children only have one father and one mother. In recent years, children could have more than 2 parents or even only one (in case of single parenthood)
The leaders of the American Revolution, who became the Founding Fathers, were heavy readers of Enlightenment literature, which convinced them to focus on reason instead of divine order.