Why would monogamous relationships be better for society from your perspective? Shouldn't you be arguing that marriage is holding our "new family unit" generation back?
I don’t believe the benefits of monogamous marriage are simply cultural. One would have to pose a better ideal for generating and raising children (because nurturing the next generation is a key part of societal survival) that leads to human flourishing. Is it polygamy, or casual hookups, or absentee fathers?
That said I do think there is more to the marriage relationship than procreation. That 2 people are becoming one. With that comes life long friendship bound by a vow that weathers the storms of life, close intimacy etc.