"St. Augustine (AD 354-430) suggested that there are three benefits of marriage: offspring (procreation), faith (fidelity), and sacrament" (from Sacred Marriage by Gary Thomas).
The word "sacrament" points to the great mystery of how God reconciles Christ to His church. Sacraments are a means of God's grace to the church. (ie: baptism, Lord's Supper) Augustine considered marriage to be a sacrament, in that it reflects the relationship between Jesus Christ and those whom he died, and is a way he extends His grace. Whether or not we continue to view marriage as a sacrament, (Protestants don't) I think we should agree that our marriages are meant to be a picture of the gospel... a picture of reconciliation. The very fact that someone is married, regardless of the quality of the relationship, signifies a spiritual union that is not meant to be seperated because it was ordained by God. It is sacred.
This ought to be a motivator for preserving our marriage and making it thrive in the midst of this fallen world. It ought to make us ask not what "we" can get out of the marriage (what makes us happy) but how our marriage can actually bring glory (happiness) to a Holy God... who instituted marriage in the garden!
Augustine's threefold view of marriage was altered by the Anglican Reformers when the phrase "mutual comfort" appeared in prayer books along with "procreation" and "fidelity". I think Augustine would say today that his view of marriage as a sacrament (a display of the gospel and a means of grace) has been muted by this change. Granted, many of us still hold our marriages to be sacred and a picture of the marriage between Christ and His church (the gospel). However, I believe the shift toward "mutual comfort" greatly effected the view of marriage.
Gary Thomas speaks about the 2 views this way...."In a man-centered view of marriage (mutual comfort), we will maintain our marriage as long as our earthly comforts, desires, and expectations are met. In a God-centered view (sacrament), we preserve our marriage because it brings glory to God and points a sinful world to a reconciling Christ."
Marriage is great when our comfort is great. The test of a marriage is when our comfort has been removed either by our own rebellion or by the rebellion of the one we married. (adultery) It is then that we realize that procreation and comfort are not enough to make up for the loss due to infidelity, which leaves us without hope. I believe Augustine would tell us that our only hope can be found in reconciliation through Christ. We must hope in the fact that our marriage is a sacred bond that represents the reality of the gospel, and although it has been violated God is able to once again reconcile and redeem what has been destroyed. (I can testify to countless couples who have experienced this reconciliation)
However, in light of the magnitude of adultery, God allows for divorce. It is not because He cannot restore, it is perhaps because He knew that the depth of pain that adultery causes is so great that mere mortals may not be able to endure.... till death do us part.
Most of us have no plans to commit such a heinous violation against our spouse and few ever set out to commit infidelity when they got married. Our charge is to live out our marriage in such a way that we care not so much for the "mutual comforts" that marriage brings (and it does!), but that our driving motivation for love is the happiness and glorification of God. It is then that we will see the passion and intent of Augustine's claim that marriage is not only for procreation and fidelity, but more importantly for the display of the gospel of Jesus Christ to a lost and fallen world.