123. After the love that unites us to God, conjugal love is the “greatest form of friendship”.(Thomas Aquinas, Summa Contra Gentiles III, 123; cf. Aristotle, Nicomachean Ethics, 8, 12 (ed. Bywater, Oxford, 1984, 174).) It is a union possessing all the traits of a good friendship: concern for the good of the other, reciprocity, intimacy, warmth, stability and the resemblance born of a shared life.
More than just friendship, marriage is properly unique among all human relationships:
Marriage joins to all this an indissoluble exclusivity expressed in the stable commitment to share and shape together the whole of life. Let us be honest and acknowledge the signs that this is the case. Lovers do not see their relationship as merely temporary. Those who marry do not expect their excitement to fade.
Two aspects: the fragility of human relationships, even marriage, and the importance of a marriage among those who accompany the couple:
Those who witness the celebration of a loving union, however fragile, trust that it will pass the test of time. Children not only want their parents to love one another, but also to be faithful and remain together. These and similar signs show that it is in the very nature of conjugal love to be definitive. The lasting union expressed by the marriage vows is more than a formality or a traditional formula; it is rooted in the natural inclinations of the human person.
Yet some skeptics remain on the naturalness of human monogamy. Even if accurate, transcending this nature remains an important witness for those who aspire to permanence.
From the Old Testament:
For believers, it is also a covenant before God that calls for fidelity: “The Lord was witness to the covenant between you and the wife of your youth, to whom you have been faithless, though she is your companion and your wife by covenant… Let none be faithless to the wife of his youth. For I hate divorce, says the Lord” (Mal 2:14-16).
Remember that Amoris Laetitia is online here.