Heidelberg Catechism - Lord's Day 41

108. Q . What does the seventh commandment teach us?
        A. That God condemns all unchastity, and that therefore we should thoroughly detest it and live decent and chaste lives, within or outside of the holy state of marriage.

109. Q . Does God, in this commandment, forbid only such scandalous sins as adultery?
        A. We are temples of the Holy Spirit, body and soul, and God wants both to be kept clean and holy. That is why God forbids all unchaste actions, looks, talk, thoughts, or desires, and whatever may incite someone to them.