The Heidelberg Catechism: Lord's Day 41
108. Question: What does the seventh commandment teach us?
Answer: That all unchastity is cursed by God. We must therefore detest it from the heart and live chaste and disciplined lives, both within and outside of holy marriage.
 Lev. 18:30; Eph. 5:3-5.  Jude 22, 23.  I Cor. 7:1-9; I Thess. 4:3-8; Heb. 13:4.
109. Question: Does God in this commandment forbid nothing more than adultery and similar shameful sins?
Answer: Since we, body and soul, are temples of the Holy Spirit, it is God’s will that we keep ourselves pure and holy. Therefore He forbids all unchaste acts, gestures, words, thoughts, desires, and whatever may entice us to
 Matt. 5:27-29; I Cor. 6:18-20; Eph. 5:3, 4.  I Cor. 15:33; Eph. 5:18.
The Heidelberg Catechism
Lord's Day 1