Catholics believe that a marriage between two baptized persons is one of the seven sacraments -- in other words, it is a human relationship which embodies and expresses God's presence and love.
As a sacrament, marriage must reflect God's relationship with humankind and Christ's relationship with the Church. That is why for Catholics a sacramental marriage must be faithful, permanent, fruitful and freely chosen -- the four fundamental characteristics of God’s relationship with humankind. Divorce, infidelity, and a deliberate intention not to accept children contradict the character of sacramental marriage.
In their Pastoral Letter Marriage--Love and Life in the Divine Plan, the U.S. bishops defined marriage as a natural institution created by God; they described the marriage relationship as “a permanent, faithful, fruitful partnership between one man and one woman, established by their mutual consent" which has two inseparable purposes: the good of the spouses and the procreation and education of children.
According to Pope Francis, the Church continues to propose this truth about marriage “not as an ideal for a few,...but as a reality that, with the grace of Christ, can be lived by all....” At the same time, the Pope has said, the Church must remember that married couples who “by free choice or unhappy circumstances live in a state of error deserve the merciful love of Christ and therefore of the Church herself.” According to the U.S. Catechism for Adults, the Church is not insensitive to the pain of the persons living in unhappy marriages. “When divorce is the only possible recourse,” the Catechism says, “the Church offers her support to those involved and encourages them to remain close to the Lord through frequent reception of the Sacraments, especially the Holy Eucharist.” (# II.21).
By granting divorced couples a Decree of Invalidity (an "annulment"), the Church recognizes that for one reason or another one or both partners in the original marriage were unable or unwilling to enter into a valid sacramental marriage which embodied the four characteristics of God's relationship with us.