Marriage Covenant (in Encyclopedia of Catholic Doctrine, Our Sunday Visitor, 1997. Ed: Russell Shaw)
The Second Vatican Council placed special emphasis on the term "covenant" to describe the unbreakable union of husband and wife (GS 48; 50). Even on the level of its natural institution, the indissolubility of marriage images the absolutely faithful character of God's love for humankind, a love which was to find very special expression in the covenant which, through Abraham (Gen 17:1-2) and Moses (Ex 19:5), he made with his Chosen People.
When Jesus Christ raised marriage to the level of a sacrament, indissolubility took on a new significance, becoming a sign of the love - faithful to death - of Christ for his Church. So indissolubility in christian marriage is said to acquire "a special firmness by reason of the sacrament" (c. 1056).
It is not correct to suggest that the use of the term "covenant" has now delegitimized reference to marriage as a contract. Both terms are applicable as appears from c. 1056, which speaks of the "matrimonial covenant" in paragraph one and of the "matrimonial contract" in paragraph two.
"Covenant" is particularly apt as drawing attention to the sacred character of marriage (even on the natural level). Its use evokes the special fidelity which should characterize conjugal love. The use of the term "contract" stresses that marriage involves the mutual interchange or rights and duties, binding in justice. It is because these rights/obligations have a juridic nature that fundamental questions related to marriage (validity of consent, separation, custody of the children, etc.) can and should be dealt with before the appropriate ecclesiastical courts.
Marriage is unique as a contract, because its nature (including its indissolubility) is determined by God. The spouses can freely choose to marry. But then they are bound to respect the sacred nature that God has given to this singular contract-covenant.