There's no logical reason why we should necessarily uphold the institution of marriage and the nuclear family as the foundational structure of society. We could

There's no logical reason why we should necessarily uphold the institution of marriage and the nuclear family as the foundational structure of society. We could all enjoy fulfilling relationships with each other and with our community's children without marriage. In fact, I'd go so far as to say that we are failing many people in society by only legitimising those relationships that are based in marriage and legal parenthood/guardianship of children. Historically, most humans did not marry and were engaged in collective child-rearing, rather than foisting the responsibility on a single pair of parents that were either biologically or legally bound to a child. These practices only emerged with the agricultural revolution. I think it's easy to imagine that children would have a far more enriching experience of growing up if they were cared for by a larger group of people with a wider range of age, experience, skills, etc. Not to mention that it would help with the problems of children growing up in "broken" or single-parent households, where they may experience poor or insufficient care and attention due to economic hardship or parental absence.