In the absence of vigorously enforced culture norms the nuclear family unit has pretty much been discarded in favor of more flexible arrangements that better fit the tastes of the individuals having the relationships. This clearly shows that the nuclear family unit is not the default smallest unit of social organization in the human species. To the contrary is is a later development that occurred in conjunction with the rise of "civilization" and it's attendant system of statism. And the human animal must be heavily socially conditioned to accept and constrain itself within this monogamous structure.
Many cannot maintain the facade long in the face of instinctive drives that evolved over millennia, which totally explains today's high rate of failure associated long term marital relationships.
The "decline in morality" that is frequently observed is nothing more than the process of discarding onerous cultural baggage that has no place in the fast paced modern world.