I'm reading Linda Waite and Maggie Gallagher's book The Case for Marriage. It's the secular/pragmatic case, one which needs desperately to be made and made well in this culture in which most people (including far too many in the church, sadly) do not recognize the authority of God's Word. Because it is secular, it will be dissatisfying to believers in several ways, but its primary message is one we can all agree is profoundly true:
Marriage is important because it gives people meaning and purpose in life by providing responsibilities that take us out of ourselves and make us care for others.
It's pretty amazing reading about just the economic, health, and personal happiness benefits of marriage that "cohabiting" couples do not receive. Waite and Gallagher make the difference clear: couples who live together do not have the permanent commitment to each other that married couples do, so they value their autonomy and live for themselves, looking towards the likely end of the relationship and how they can be prepared for it. Married couples, by virtue of their commitment, look for ways to make things work and have the freedom to depend on each other.
Amazing how God's ways work even when we don't acknowledge they are His.