God Himself is the author of marriage. He began by creating man and woman. Thus, in the very nature of man and woman, the vocation to marriage is written, i.e. a partnership of the whole of life and by its nature ordered toward the good of the spouses and the procreation and education of their offspring. (CCC 1601-1603) 

Holy Scripture affirms that man and woman were created for one another: “It is not good that the man should be alone.” The woman, “flesh of his flesh,” i.e., his counterpart, his equal, his nearest in all things, is given to him by God as a “helpmate”; she thus represents God from whom comes our help. Therefore, a man leaves his father and his mother and cleaves to his wife, and they become one flesh. The Lord himself shows that this signifies an unbreakable union of their two lives by recalling what the plan of the Creator had been “in the beginning”: “So they are no longer two, but one flesh. (CCC 1605) 

God who created human out of love, also calls them to love, i.e. the fundamental and innate vocation of every human being: loving God and loving one another. For human is created in the image and likeness of God who is Himself love. (CCC 1604) 

Indeed, 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. These differences should not cause us to forget its common and permanent characteristics, i.e. the unity and indissolubility, fidelity, and openness to fertility. Although the dignity of this institution is not transparent everywhere with the same clarity, some sense of the greatness of the matrimonial union exists in all cultures, i.e. the well-being of the individual person and of both human and of society, closely bound up with the healthy state of conjugal and family life.” (CCC 1603)