"Thou shalt not commit adultery."

The duties required by the seventh commandment are,
-chastity in body, mind, affections, words, and behavior; and
-the preservation of it in ourselves and others;
-watchfulness over the eyes and all the senses;
-temperance; keeping in chaste company; modesty in apparel;
-marriage by those that have not the gift of continency;
-conjugal love, and cohabitation;
-diligent labor in our callings;
-shunning all occasions of uncleanness, and resisting temptation thereunto. (taken from the Larger Catechism of the Presbyterian Church - Answer to Question 138)

Commanded:  Loving and being faithful to your spouse with the highest regard for the sanctity of the God-ordained institution of marriage.  Loving others with a sexually pure, chaste love, striving to preserve and promote their chastity of heart and behavior as well as your own.

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 and impudent or light behavior;
-prohibiting of lawful, and dispensing with unlawful marriages;
- allowing, tolerating, keeping of (brothels), and resorting to them;
-entangling vows of single life;
-undue delay of marriage;
-unjust divorce or desertion or 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. (taken from the Larger Catechism of the Presbyterian Church - Answer to Question 139).

Forbidden:  Self-indulging in sexual acts and impure thoughts (outside of the God-ordained state of marriage).  Treating the institution of marriage lightly; indulging in anything, even imaginations, that tend to defile yourself, or in questionable behavior or speech that might stimulate others to lustful feelings.