Catechism of the Catholic Church

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. The sixth commandment and the New Testament forbid adultery absolutely. The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry. (1611)