Marriage is about the covenant of love which two people make toward one another; giving themselves totally, exclusively, and perpetually to the other person. When we think "marriage" this is the image that often comes to mind, at least for a "theologically updated" Catholic.
However the sacrament of marriage is something else. The sacrament of marriage is about God. To use the metaphor in the title of Martos's book, sacraments are "doors to the sacred", they enable us to pass from this world into the divine reality. Perhaps and even more intimate metaphor which I prefer, even more than "doors to the sacred", they are "glimpses through the window into God's bedroom". We spy on the very loving activity of God! And we become so desirous of that intimate Trinitarian act of loving that with our whole heart we strive to pass from this life into that eternal, divine, loving– which the Theologians have named divinization, or in Greek (as in the Eastern tradition), theosis.
Throughout this course when we speak of the sacrament of marriage I presume that this is what you are thinking about.
When we see the couple there in front of us during the celebration of the sacrament proclaiming their perpetual love to one another, what do we see? Simply the husband-and-wife? Simply the marriage? Or are we receiving a sacrament, are receiving a glimpse of who God is!
As they promise to love one another through thick and thin, in sickness and in health, in good times and bad, ... I get a glimpse of how God loves me: in sickness and in health in good times and in bad, when I'm performing virtuous acts and when I am sinning.
In the sacrament of marriage, when I see the couple pledge their love, not a 50-50 exchange but an all or nothing covenant. It's not "I will love you if you love me back", but it's, "I will love you no matter what you do". The sacrament of marriage tells me that's how God loves me.
Think on this: try to find ten, fifteen, or twenty things that the sacrament of marriage tells us about God.
As St. Augustine said "sacraments are visible sign of the invisible God". That's how we want to think about sacraments. Not simply as a "pill" that we take to receive grace. That is an "item of furniture" that I hope you remove from your theological "living room."
I hope that you can replace this "shoebox" era furniture with much more modern and comfortable items such as "sacraments are persons, not things" – Sacraments are not something we receive, they are something we are. Sacraments are verbs not nouns.
Marriage is a sacrament we receive each time we witness a couple exchange their vows. I hope that each of us– married, unmarried, priests, deacons, sisters, widows,– have received and continue to receive these graces of the sacrament many, many times.