The textbook Marriage and Holy Orders: Your Call to Love and Serve addresses the unity and indissolubility of marriage. For example:

In marriage, the man and woman grow continually in their life together through their daily fidelity to one another and through their mutual self-giving. The root of this relationship is the natural complementarity that exists between a man and woman. God made both man and woman for each other. But even more than this natural complementarity that keeps the couple together is the gift of the Holy Spirit given to them in the sacrament of Matrimony. Pope John Paul II wrote of this gift:

The gift of the Spirit is a commandment of life for Christian spouses and at the same time a stimulating impulse so that every day they may progress towards an even richer union with each other on all levels—of the body, of the character, of the heart, of the intelligence and will, of the soul—revealing in this way to the Church and to the world the new communion of love, given by the grace of Christ (Familiaris Consortio. 19).

This unique relationship of spouses is characterized not only by unity but also by its indissolubility. A marriage demands absolute fidelity between spouses. As the Catechism teaches: “Love seeks to be definitive; it cannot be an arrangement ‘until further notice’” (CCC, 1646). Fidelity is not just a matter of being physically faithful to one’s spouse. Fidelity also requires that one be faithful in one’s heart too. Indulging in daydreams about another person or about life without one’s spouse is an occasion of unfaithfulness. Fidelity to one other person may seem impossible; there are, after all, many people in the world whom you can be attracted to and infatuated with, even after you are married. But that is why God instituted the sacrament of Matrimony. The sacrament allows a couple to do what is impossible:

The gift of the sacrament is at the same time a vocation and a commandment for the Christian spouses, that they may remain faithful to each other forever, beyond every trial and difficulty, in generous obedience to the holy will of the Lord: “What therefore God has joined together, let man not put asunder” (Familiarias Consortio, 20, quoting Matthew 19:6).

The graces of the sacrament face strong and severe threats today against the unity and indissolubility of marriage. As part of a lesson, play the students a audio or video performance of Bruce Springsteen’s “If I Should Fall Behind” in which he sings

We swore we’d travel darlin’ side by side
We’d help each other stay in stride
But each lover’s steps fall so differently
But I’ll wait for you
And if I should fall behind
Wait for me.

Have your students comment on the lyrics and discuss what they have to do with marriage. Also ask:

What does it mean to say that marriage is perpetual and exclusive?
What is the meaning of fidelity in marriage?
* Share an example of a couple that exemplifies a strong marriage, including times when one spouse waited for and helped the other through a dark time.