Q. 1. What is the ninth Commandment and what are the sins against it?
A. 1. The ninth Commandment is, “You shall not covet your neighbour's wife.” (To covet means to desire, to long for, to aim after.)
What is forbidden by this Commandment? All sensual, impure thoughts and desires are forbidden; such things are sinful if they are wilfully and deliberately kept in one’s mind.
“Every one who looks at a woman lustfully has already committed adultery with her in his heart.” [Mt. 5:28] “Evil plans are an abomination to the Lord, but gracious words are pure.” [Prov. 15:26]
Are involuntary, impure thoughts sinful? No. But we should strive to banish such thoughts as soon as we realize their nature, and turn our attention to other things.
The heart is the seat of moral personality: "Out of the heart come evil thoughts, murder, adultery, fornication...." [Mt 15:19] The struggle against carnal covetousness entails purifying the heart and practicing temperance. (C.C.C. # 2517)