Shorter Catechism Q. 72. What is forbidden in the seventh commandment? A. The seventh commandment forbiddeth all unchaste thoughts, words and actions.
Commentary: The seventh commandment forbids “all unchaste thoughts.” Jesus says that we break this commandment in our hearts if we look at someone with lustful intent (Matt. 5:28; Prov. 6:25). This means that we are not to fantasize about having sexual relations with someone who is not our spouse.
This commandment also forbids “all unchaste…words.” We shouldn’t flirt or sext with someone who is not our spouse. Nor should we positively talk about immoral fantasies, including so-called “hall passes.”
Still further, this commandment forbids “all unchaste…actions.” This includes physical adultery, premarital sex (e.g., sex after engagement but prior to marriage), sex outside of marriage, homosexuality, watching pornography, and other forms of sexual immorality short of intercourse.
WLC 139 provides a detailed list of what this commandment forbids: “The sins forbidden in the seventh commandment, besides the neglect of the duties required, are adultery, fornication, rape, incest, sodomy, and all unnatural lusts; all unclean imaginations, thoughts, purposes, and affections; all corrupt or filthy communications, or listening thereunto; wanton looks, impudent or light behaviour, immodest apparel; prohibiting of lawful, and dispensing with unlawful marriages; allowing, tolerating, keeping of stews [brothels/prostitutes], and resorting to them; entangling vows of single life, undue delay of marriage; having more wives or husbands than one at the same time; unjust divorce, or desertion; idleness, gluttony, drunkenness, unchaste company, lascivious songs, books, pictures, dancings, stage plays; and all other provocations to, or acts of uncleanness, either in ourselves or others.”