Christophera
Evidence & Reason Rule
I don't agree with one of your premises - I don't think that the percentage of married couples is an indicator of whether or not children have both parents in their lives - many couples with children that I know have never bothered to get married yet still live together and raise their children together - as well as couples who are no longer together yet share custody of their children.
Marriage is not the same as a child having two parents in their lives.
You are correct with relationship to children within the sphere of the immediate family.
The role of marriage that is missing relates to the natural law responsibility of fertility in society. None get the benefit of that in its greater function which was political.