Marriage is the only sacrament in which the minister is not an ordained minister, but the spouses themselves, at the moment when they exchange consent before God, the Church, and the community.
The priest accepts their consent and gives the Church’s blessing.
The Catechism of the Catholic Church defines marriage as, “The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament” (#1601).
Therefore; a valid celebration of matrimony creates a perpetual and exclusive bond between the spouses. The exclusive union of one man and one woman is a “natural law” written in human nature by the Creator from the beginning, and common to humankind at every time, in every people and culture.