Catechism of the Catholic Church

This is the reason why the Church normally requires that the faithful contract marriage according to the ecclesiastical form. Several reasons converge to explain this requirement: (1069, 1537, 2365) - Sacramental marriage is a liturgical act. It is therefore appropriate that it should be celebrated in the public liturgy of the Church; - Marriage introduces one into an ecclesial order, and creates rights and duties in the Church between the spouses and towards their children; - Since marriage is a state of life in the Church, certainty about it is necessary (hence the obligation to have witnesses); - The public character of the consent protects the "I do" once given and helps the spouses remain faithful to it.