Why Get Married?
De facto unions, the number of which is progressively increasing, are based on a false conception of an individual's freedom to choose and on a completely individualistic vision of marriage and family. Marriage is not a simple agreement to live together but a relationship with a social dimension that is unique with regard to all other relationships, since the family — attending as it does to caring for and educating children — is the principal instrument for making each person grow in an integral manner and integrating him positively into social life.
Making “de facto unions” legally equivalent to the family would discredit the model of the family, which cannot be brought about in a precarious relationship between persons but only in a permanent union originating in marriage, that is, in a covenant between one man and one women, founded on the mutual and free choice that entails full conjugal communion oriented towards procreation.
Compendium of the Social Doctrine of the Church,