Obviously, the first men had to take their siblings and children as spouses, though this is denied to us now. "Just as this is the best thing to do when natural necessity compels it, it becomes all the more wicked when moral obligation condemns it."
Augustine gives us evidence of this:
This can be proved as follows. The supreme human law is love and this law is best respected when men, who both desire and ought to live in harmony, so bind themselves by the bonds of social relationships that no one man monopolizes more than one relationship, and many different relationships are distributed as widely as possible, so that a common social life of the greatest number may best be fostered.That is, when there are only a few people in the world, we have to just love who we can as best we can. When the number grows, we are bound to go outside of our families to love others well. "Thus, once there was no necessity for the old arrangement, it ceased to have any moral validity." Even custom and tradition get on board with this new state of affairs:
In general, custom has great power both in provoking and preventing the play of human passion. In this matter, custom keeps concupiscence in bounds and, therefore, any detraction from or destruction of custom is branded as criminal. Thus, unjust as it is to encroach, out of greed, on another's property, it is still more wicked to transgress, out of lust, the limits of established morals.Thus, the foundation of the earthly city is certainly this act of marriage and child-rearing. Yet, the foundation of the heavenly City is regeneration:
The union of male and female is, then, so far as mortal living goes, the seed-bed, so to speak, from which a city must grow; but, while the city of earth needs only human generation, the City of heaven demands a spiritual regeneration to escape from the taint of the generative act.While Augustine perhaps puts too much of the weight of original sin on the sexual act, his point is still a good one. The physical and natural continuation of the human species is necessary to the flourishing of the city of man--just try having an underpopulated nation and see where it gets you! However, no one enters that way into the Kingdom of God. That comes by faith, which is the result of grace converting the soul away from self worship and to belief in the Gospel of Jesus Christ.