The spouses' union achieves the twofold end of marriage: the good of the spouses themselves and the transmission of life. These two meanings or values of marriage cannot be separated without altering the couple's spiritual life and compromising the goods of marriage and the future of the family.
The conjugal love of man and woman thus stands under the twofold obligation of fidelity and fecundity.
Source: Catechism of the Catholic Church (Vatican.va)