19 The second blessing of matrimony which We said was mentioned by St. Augustine, is the blessing of conjugal honor which consists in the mutual fidelity of the spouses in fulfilling the marriage contract, so that what belongs to one of the parties by reason of this contract sanctioned by divine law, may not be denied to him or permitted to any third person; nor may there be conceded to one of the parties anything which, being contrary to the rights and laws of God and entirely opposed to matrimonial faith, can never be conceded.
Source: Casti Connubii (Vatican.va)