The Norwegian novelist, Sigrid Undset (1882-1949), baptized a Lutheran but raised by agnostic parents, and who emerged from a difficult seven-year marriage at age thirty-seven, converted to Catholicism in 1924. In 1928, she received the Nobel Prize for Literature. Shortly after, a priest in Oslo asked her why, even before her conversion, she had referred to marriage as a “sacrament” in her novels of medieval Norway, although for a Protestant marriage is only a contract.
She replied that this would require a rather lengthy explanation, and she offered the explanation in an essay published in an Oslo Catholic magazine, Credo, which was later included as a chapter in her Stages on the Road  (1934), and in 2006 reprinted in Through Moral Crises to Catholicism (Reply to a Parish Priest). with an Introduction by the late Fr. Stanley Jaki.
She writes that she had tended to regard all Christian marriage as having a sacramental character, but gradually came to realize this was not the case:
It seemed to me and to many who shared my views that logically it must be a dogma common to all evangelical Christianity, that for Christians there could be no question of anything but lifelong, indissoluble monogamy as the only permissible form of marriage. But we were faced with the historical fact that all the founders of Protestant sects had agreed in throwing this dogma overboard. They had all accepted the view that in certain circumstances marriage may be dissolved and that divorced persons may marry again, even during the lifetime of their former partner. Luther had flirted freely with the idea of polygamy and permitted persons of rank at any rate to take a secondary wife.
As I have mentioned in a previous article , one of the “unnecessary” sacraments that Luther and other reformers abrogated in the name of Christian freedom was the sacrament of matrimony, which was relegated largely to civil law rather than Church law. In Catholic doctrine, this sacrament is conferred not by a priest, but by a man and woman in the presence of a priest; and, if and when they are in the state of grace, offers special graces to the couple for a lifelong relationship that mirrors the mystical wedding of Christ and the Church. (Eph. 5:32) Undset found it hard to believe that Protestants would give short shrift to this sacramental aspect and treat marriage just as one other contract subject to civil laws:
As a sacrament – a means of grace – marriage must have been instituted primarily to help people on the road to eternal salvation. On no other assumption is it at all likely that men could ever have claimed that it is, and must be, an indissoluble union, in which both parties in the first place undertake duties towards God, and towards each other in God. Even while the Church’s doctrine of marriage was held to be objectively true and right practically all over Europe, adultery was quite an everyday occurrence; but the Church could say with full justification – marriage is a means of grace, but if men refuse to co-operate with grace, it is no use; men have none the less their free will to sin.
Sigrid Undset in 1928
Fr. Henry Sattler has explained  that sacramental grace in marriage “is the special mode of Sanctifying Grace which makes the receiver a habitual connatural principle of supernatural action in Marriage – which means that the love, and love-making, and housekeeping and work and worry of marriage are all deified.”
In other words, sacramental married life is not just a fulfillment of contractual arrangements, but a special vocation with supernatural assistance and supernatural significance provided continually to partners in the state of grace.
Theologians are divided as to just how the sacrament of marriage contains grace (ex opere operato). The sacrament of baptism frees the soul from original sin; the sacrament of Holy Orders bestows a special character on the recipient. If a partner with serious unconfessed sin enters into the sacrament of marriage, the act is sacrilegious; but confession and forgiveness of sin makes him or her eligible for all the graces connected with the marriage.
Sigrid Undset ends her essay with an appeal to fellow Catholics to be aware that European traditions, including traditions of marriage, have been derived from Catholic sources, but have in several respects evolved into pale imitations:
My intention in writing this article is in the first place this – to beg Catholics, here in Scandinavia and elsewhere, to understand that it cannot be otherwise. We must try to make this clear to ourselves – we have no right to assume that any part of European tradition, cultural values, moral ideas, emotional wealth, which has its origin in the dogmatically defined Christianity of the Catholic Church, will continue to live a “natural” life, if the people of Europe reject Christianity and refuse to accept God’s supernatural grace. One might just as well believe that a tree whose roots were severed should continue to bear leaves and blossoms and fruit.
Sigrid Undset presumably was not yet caught up in the social currents advocating gay “marriage” and polygamy, but if marriage is just a contract, such developments are quite conceivable. If, however, marriage is a sacrament mirroring and perpetuating the eternal espousal of Christ and the Church, such “extensions” are clearly sacrilegious.