In a strict nuclear family model, parents are left without support from outside adults. They are left to work and raise children in isolation, depending only upon their partner for support.
Now let me just ask you, what kind of relationship can any of us have without outside relational support? Even in my relationship with my best friend, I receive support from other friends. If I couldn’t talk to other friends about conflicts he and I encounter, I would probably say vastly more stupid things than I do already.
Additionally, no one person can fulfill all of your relational needs. A spouse, no matter how selfless, how giving, how beautiful, or how patient they may be, cannot be all that you need them to be. The idolization of the nuclear family restricts the amount of support married couples are able to receive and takes a heavy tole on their sacred relationship.