The Baltimore Catechism
382. Q. What is the Ninth Commandment?
A. The Ninth Commandment is: Thou shalt not covet thy neighbor’s wife.
A. We are commanded by the Ninth Commandment to keep ourselves pure in thought and desire.
A. The Ninth Commandment forbids unchaste thoughts, desires of another’s wife or husband, and all other unlawful impure thoughts and desires.
A. Impure thoughts and desires are always sins, unless they displease us and we try to banish them.