The vows of a valid, licit marriage demand the full gift of the man for the woman and the woman for the man and thus mention "until death do us part" in order to define what that full gift of self means. Any agreement which contemplates a future possible termination of the union short of death, therefore, vitiates the full gift of self necessary for a valid marriage.
The Catechism has this to say:
"The Lord Jesus insisted on the original intention of the Creator who willed that marriage be indissoluble. He abrogates the accommodations that had slipped into the old Law. Between the baptized, 'a ratified and consummated marriage cannot be dissolved by any human power or for any reason other than death.' " (CCC 2382)
"Divorce is a grave offense against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other till death. Divorce does injury to the covenant of salvation, of which sacramental marriage is the sign. Contracting a new union, even if it is recognized by civil law, adds to the gravity of the rupture: the remarried spouse is then in a situation of public and permanent adultery:
Godly love and grace make the tremendous and beautiful vows of marriage possible and desirable:
"Seeing God's covenant with Israel in the image of exclusive and faithful married love, the prophets prepared the Chosen People's conscience for a deepened understanding of the unity and indissolubility of marriage. The books of Ruth and Tobit bear moving witness to an elevated sense of marriage and to the fidelity and tenderness of spouses. Tradition has always seen in the Song of Solomon a unique expression of human love, insofar as it is a reflection of God's love - a love 'strong as death' that 'many waters cannot quench.' " (CCC 1611)