IV. Offenses Against the Dignity of Marriage
Catechism number 2380
Adultery refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations - even transient ones - they commit adultery. Christ condemns even adultery of mere desire.170 The sixth commandment and the New Testament forbid adultery absolutely.171 The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.172