The Catechism of the Catholic Church, in the section about the sacrament of marriage, states:
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.'"
1605 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," 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."
Regarding your original concern, a secular vision of marriage as merely a human-made, institutionalised contract may try to deny marriage in Adam and Eve by pointing out the lack of an explicit contract. This is a total reversal of the actual logic of marriage. Marriage is the outcome of a blessing, of spouses subduing to God's will. The institutionalised contract is merely a reflection of this outcome.
