Allan Carlson, one the leading conservative marriage proponents, uses this phrase in his new book, Conjugal America. The foundation of this unwritten constitution, he argues, is procreative marriage. Tying procreation and marriage firmly together is, he argues, the cultural achievement on which the rest of Western civilization rests.
Carlson frames the creation of the unwritten sexual constitution in the chaos at the end of the Roman empire. The ancient Roman republic had had a stern marriage code. By the early years of the church, though, the decadence of empire had eroded that old standard to an "anything goes" sexual culture. Making babies and being married had become disconnected.
In the first centuries of the Christian era, Carlson argues, "the Fathers of the Christian Church crafted a new sexual order." They were arguing not only with pagans, but also with two Gnostic strands within Christianity. The Gnostics thought they had a secret knowledge that set them apart from ordinary people. One Gnostic group took their specialness to mean that no sexual rules applied to them, that gospel freedom meant all sex was fine. The other, more intellectually potent brand of Gnosticism argued that sex and procreation were bad, that living in our material bodies is a trial and an imprisonment. On the one hand, sex and babies without marriage. On the other hand, no sex and no babies.
The leaders of the early church took a middle path. They saw, as Genesis says, that Creation is good. Babies are good, and thus sex has its place. The great achievement of the early church, Carlson argues, is to promote an ideal that put sex in marriage for making and raising children. Children, as his first chapter title proclaims are the "first purpose of marriage." Carlson credits Augustine with the most enduring formulation of the Christian ideal of procreative marriage. We sometimes think of Augustine as anti-sex, but really he was promoting a high standard of sex within marriage because sex has such a strong potential to lead us astray.
"And," Carlson writes, "as articulated by Augustine in the year 400 AD, this moral order lasted for another 1,500 years."
I think the idea that there is an unwritten sexual constitution to our civilization based on marriage and children is a powerful one. In fact, I would go a step further, and argue that procreative marriage is Article I of the unwritten constitution of every civilization.