Catechism of the Catholic Church

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. ¹⁷¹ The sixth commandment and the New Testament forbid adultery absolutely. ¹⁷² The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry. ¹⁷³

2381 Adultery is an injustice. He who commits adultery fails in his commitment. He does injury to the sign of the covenant which the marriage bond is, transgresses the rights of the other spouse, and undermines the institution of marriage by breaking the contract on which it is based. He compromises the good of human generation and the welfare of children who need their parents' stable union.