Q: What is forbidden in the seventh commandment?
A: The seventh commandment forbiddeth all unchaste thoughts, words, and actions.

Matthew 5:28. But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Ephesians 5:4. Neither filthiness, nor foolish talking, nor jesting, which are not convenient: but rather giving of thanks.
Ephesians 5:3. But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints.

Westminster Project - See John Flavel for further thinking on the seventh commandment