The Catechism in Questions & Answers

344. What is the significance of the Sixth Commandment in the New Testament?

Jesus Christ gives unequivocal support to monogamy (marriage to one partner). This is the form of matrimonial cohabitation of man and woman desired by God and appropriate to believing Christians.
Jesus also expanded the interpretation of the Sixth Commandment beyond its original meaning. In the Sermon on the Mount He said: “But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart” (Matthew 5: 28). This means that “adultery of the heart”, in other words, adultery played out in thought, can occur despite an outwardly blameless conduct of life.

‘Monogamy’ describes the situation where a man is married to only one woman and a woman is married to only one man.—The Old Testament often speaks of ‘polygamy’ (marriage to several partners) in the sense that one man was married to several women.