It does sound harsh, I agree, but I’m afraid that is exactly what it means. God is not a part of the marriage, as in a sacramental marriage, and therefore, it is not blessed by God. A sacramental marriage is a holy marriage, meaning blessed by God, just as water blessed by God through a priest is holy, and water that is not blessed is not holy. It’s still water, but it ain’t holy.
Your statement is at odds with what St. Paul teaches in 1 Cor 7:14:
For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy.
A marriage in which one or both parties is unbaptized does not receive *sacramental *grace, a.k.a. sanctifying grace . But that’s not the only kind of grace. Couples in a natural marriage can receive actual grace.
The catechism also refutes your concept that marriage of the unbaptized is purely of the human order having no divine dimension. The CCC describes marriage as being in the order of Creation, from the very beginning. It is raised to a Sacrament among the baptized, but for the unbaptized it is still an institution ordained by, and blessed by, God:
1603 “The intimate community of life and love which constitutes the married state has been established by the Creator and endowed by him with its own proper laws. . . . God himself is the author of marriage.” The vocation to marriage is written in the very nature of man and woman as they came from the hand of the Creator. **Marriage is not a purely human institution **despite the many variations it may have undergone through the centuries in different cultures, social structures, and spiritual attitudes.
1604 God who created man out of love also calls him to love the fundamental and innate vocation of every human being. For man is created in the image and likeness of God who is himself love. Since God created him man and woman, their mutual love becomes an image of the absolute and unfailing love with which God loves man. It is good, very good, in the Creator’s eyes. **And this love which God blesses **is intended to be fruitful and to be realized in the common work of watching over creation: "
And God blessed them, and God said to them: ‘Be fruitful and multiply, and fill the earth and subdue it.’"