Vatican II, "Gaudium et spes"

The holiness of married life and family life

A man and a woman, who by the marriage covenant of conjugal love ‘are no longer two, but one flesh’, render mutual help and service to each other through an intimate union of their persons and of their actions.
  Through this union they experience the meaning of their oneness and attain to it with growing perfection day by day. As a mutual gift of two persons, this intimate union, as well as the good of the children, imposes total fidelity on the spouses and argues for an unbreakable oneness between them.
  Christ the Lord abundantly blessed this many-faceted love, welling up as it does from the fountain of divine love and structured as it is on the model of his union with the Church.
  As God of old made himself present to his people through a covenant of love and fidelity, so now the Saviour of men and the spouse of the Church comes into the lives of married Christians through the sacrament of matrimony. He abides with them thereafter so that, just as he loved the Church and handed himself...