The Book of Tobit has for its subject the providence of God, working for the good of his handmaids and servants.  Specifically, Tobiah and Sarah are brought together for marriage, according to God's providential design.  In his essay, "On the Good of Marriage", St. Augustine reflects that matrimony has its origin in the very nature of the human person, as created by Almighty God at the beginning of the world:

"Because each man is a part of the human race, and humanity is social by nature, the virtue of friendship is man's first great and natural good.  Because of this, God willed to make all men from one so that they might be bound not only by similitude of kind, but also by the bond of blood-relation.  Therefore, the first natural tie of human society was that of a man and a woman.  God did not create them as individual persons, joining them as strangers to one another, but he created the one out of the other.  The power of this joining together is signified in the side of the one from whom the other was taken (Genesis 2, 21-22).  They are joined to each other, walk together side by side, and look together to where they are walking.  A consequence of this is the connection of society in their children."

The creation of the human race was not accomplished fully until Adam and Eve were created out of one another, to be joined in marriage and oriented for children by God.  Thus, the nature of marriage as between a man and a woman comes from the social nature of the human person.