Heidelberg Catechism – Lord’s Day 41
Q. What does the seventh commandment teach us?
A. That all unchastity is condemned by God, and that we should therefore detest it from the heart, and live chaste and disciplined lives, whether in holy wedlock or in single life.
Q. Does God forbid nothing more than adultery and such gross sins in this commandment?
A. Since both our body and soul are a temple of the Holy Spirit, it is his will that we keep both pure and holy. Therefore he forbids all unchaste actions, gestures, words, thoughts, desires and whatever may excite another person to them.