The Baltimore Catechism

382. Q. What is the Ninth Commandment?

A. The Ninth Commandment is: Thou shalt not covet thy neighbor's wife.

383. Q. What are we commanded by the Ninth Commandment?

A. We are commanded by the Ninth Commandment to keep ourselves pure in thought and desire.

384. Q. What is forbidden by the Ninth Commandment?

A. The Ninth Commandment forbids unchaste thoughts, desires of another's wife or husband, and all other unlawful impure thoughts and desires.

385. Q. Are impure thoughts and desires always sins?

A. Impure thoughts and desires are always sins, unless they displease us and we try to banish them.