Exodus 20:14

Thou shalt not commit adultery - Adultery, as defined by our laws, is of two kinds; double, when between two married persons; single, when one of the parties is married, the other single. One principal part of the criminality of adultery consists in its injustice.

1. It robs a man of his right by taking from him the affection of his wife.

2. It does him a wrong by fathering on him and obliging him to maintain as his own a spurious offspring - a child which is not his. The act itself, and every thing leading to the act, is prohibited by this commandment; for our Lord says, Even he who looks on a woman to lust after her, has already committed adultery with her in his heart. And not only adultery (the unlawful commerce between two married persons) is forbidden here, but also fornication and all kinds of mental and sensual uncleanness. All impure books, songs, paintings, etc., which tend to inflame and debauch the mind, are against this law, as well as another species of impurity, for the account of which the reader is referred to; See Clarke's note on Genesis 38:30.

That fornication was included under this command we may gather from St. Matthew, Matthew 15:19, where our Savior expresses the sense of the different commandments by a word for each, and mentions them in the order in which they stand; but when he comes to the seventh he uses two words, μοιχειαι πορνειαι, to express its meaning, and then goes on to the eighth, etc.; thus evidently showing that fornication was understood to be comprehended under the command, "Thou shalt not commit adultery." As to the word adultery, adulterium, it has probably been derived from the words ad alterius torum, to another's bed; for it is going to the bed of another man that constitutes the act and the crime. Adultery often means idolatry in the worship of God.

20:14 Thou shalt not commit adultery - This commandment forbids all acts of uncleanness, with all those desires, which produce those acts and war against the soul.