• Dr. Ray E. Heiple, Jr.

What Sexual Sin Is

Marriage is honorable among all, and the marriage bed undefiled; but fornicators and adulterers God will judge. Hebrews 13:4NKJV


Today we begin to look at Question 139 of the Larger Catechism, which asks, “What are the sins forbidden in the seventh commandment?” The first part of the answer states, “The sins forbidden in the seventh commandment, besides the neglect of the duties required, are adultery, fornication, rape, incest, sodomy, and all unnatural lusts…” Last week we examined our duty to avoid all occasions and temptations to sexual sin. This week we consider those forms of sexual activity which are forbidden by the seventh commandment for the glory of God and the good of man.


Previously we observed the fact that sex is the idea and creation of God. Although some single-celled organisms and plants were created by God to reproduce asexually, God made most of the creatures, including mankind, sexual creatures. Sexual creatures are those kinds of creatures which have both males and females within their particular species. The maleness or femaleness of a creature can be seen in a variety of different physical characteristics, which are sometimes present between the two sexes, such as: different sizes and shapes, distinct color patterns or brightness, the presence of a mane or of facial and chest hair, the presence or size of antlers or horns, etc. However, male and female physical distinctiveness is always found in the different male and female sex organs of a particular kind of creature. In human beings as well as in many of the animals, male and female sex organs are designed conjugal to one another, meaning they are made to be joined or “yoked” together in the act of sex.


Man, having been made in the image and likeness of God, must participate in the act of sex in a way that glorifies God. As has been previously seen, sex is glorifying to God when it is between one man and one woman who have been united in the covenant of marriage. Sex in marriage is commanded by God and glorifies God. Moreover, as in all acts of obedience to God, sex in marriage is good for the couple. It is good physically, mentally, and emotionally. Similarly, when man engages in sinful sexual activity, in disobedience to God, it is harmful to man. The Catechism lists the following specific sinful forms of sex:

  1. Adultery is when one person in the marriage relationship has sex with someone not his or her spouse. In a sense adultery is the grossest form of sexual sin because it breaks the deepest bond of trust and intimacy that two human beings can have with one another: marriage.

  2. Fornication is when two unmarried people have sex with one another. This is a sin because they must formally commit to each other in marriage before they join in this most intimate of ways. A society that condones or overlooks the sin of fornication cheapens the act of sex as something less than the holy expression of the love of a husband and wife. Such a society will quickly degenerate into all kinds of sexual depravity and lewdness.

  3. Rape is when a person, usually a woman, is forced to have sex against her will. Rape humiliates a person in the greatest possible way and should be abhorrent to all human beings. Thankfully, our sexually depraved society still “judges” rapists as evildoers.

  4. Incest is when two near relatives have sex with one another. When God first created mankind it was natural for brother to marry sister. However once the race expanded that necessity no longer existed and it was banned (Deut. 27:20-23). The Bible always forbade children from lying with their parents, proving that incest as a moral concept is part of man’s moral nature.

  5. Sodomy is the sin of males having sex with males or females with females. It is euphemistically called “homosexuality” today, but it is just as wicked in the eyes of God. As noted above two males or two females cannot be joined together by their sex organs. They can experience sex only through an unnatural perversion of their sex organs. Thus, sodomy or homosexuality is dishonoring to God and shameful to man. It is the only form of sexual sin explicitly named by God as a sin against nature (Rom. 1:26-27).

  6. Consequently, the Catechism forbids all unnatural lusts immediately following the sin of sodomy. Here would fall every other kind of sexual activity and desire that can possibly be imagined by the wicked heart of man. The Bible names the perversion of lying with animals as one such possibility (Deut. 27:21), and our society has added many others we need not name.

The harm from these forms of sexual sin should be self-evident. Yet, we need only to look at our world today to see that God’s laws are good. When we turn from them, we turn to our own ruin.

A member of the Presbyterian Church in America (PCA)

Reformed doctrine. Reverent worship. Real life.

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