What does the seventh commandment teach us?

Heidelberg Catechism (extended)

The Catechism
Method of Instruction in the Christian Religion
As the Same is Taught in the Reformed Churches and Schools

(with the Scripture references written out)

Note. This Catechism is fully based on the Scriptures. The references to Scripture are indicated in parentheses with a letter. For example, the letter (a) points to the texts (a) placed after the answer.

41. Lord's Day 

Q. 108. What does the seventh commandment teach us? 

A. That all uncleanness is accursed of God: (a) and that therefore we must with all our hearts detest the same, (b) and live chastely and temperately, (c) whether in holy wedlock, or in single life. (d) 

(a) Lev.18:27 (For all these abominations have the men of the land done, which were before you, and the land is defiled;) Lev.18:28 That the land spue not you out also, when ye defile it, as it spued out the nations that were before you. 

(b) Jude 1:23 And others save with fear, pulling them out of the fire; hating even the garment spotted by the flesh. 

(c) 1 Thess.4:3 For this is the will of God, even your sanctification, that ye should abstain from fornication: 1 Thess.4:4 That every one of you should know how to possess his vessel in sanctification and honour; 1 Thess.4:5 Not in the lust of concupiscence, even as the Gentiles which know not God: 

(d) Heb.13:4 Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge. 1 Cor.7:7 For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that. 1 Cor.7:8 I say therefore to the unmarried and widows, It is good for them if they abide even as I. 1 Cor.7:9 But if they cannot contain, let them marry: for it is better to marry than to burn. 1 Cor.7:27 Art thou bound unto a wife? seek not to be loosed. Art thou loosed from a wife? seek not a wife. 

Q. 109. Does God forbid in this commandment, only adultery, and such like gross sins? 

A. Since both our body and soul are temples of the holy Ghost, he commands us to preserve them pure and holy: therefore he forbids all unchaste actions, gestures, words, (a) thoughts, desires, (b) and whatever can entice men thereto. (c) 

(a) Eph.5:3 But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints; Eph.5:4 Neither filthiness, nor foolish talking, nor jesting, which are not convenient: but rather giving of thanks. 1 Cor.6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. 1 Cor.6:19 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? 1 Cor.6:20 For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's. 

(b) Matt.5:27 Ye have heard that it was said by them of old time, Thou shalt not commit adultery: Matt.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. 

(c) Eph.5:18 And be not drunk with wine, wherein is excess; but be filled with the Spirit; 1 Cor.15:33 Be not deceived: evil communications corrupt good manners.

Source: http://www.ccel.org/creeds/heidelberg-cat-ext.txt