Once again, the biblical context is much more restrictive than our contemporary appropriation and understanding. Technically speaking, sexual impropriety, immorality, or fornication is not in view here (see chapters 22-25). Wives were prohibited from sexual intercourse with any male other than their husband. Husbands were prohibited from sexual intercourse with any married woman other than their wife. This was to protect the paternity of any ensuing offspring. Part of the difficulty with our appropriation of this commandment lies in our very different understandings of marriage. Israelite society permitted both polygamy and concubines, which we do not. In addition, husbands were not obliged to be "faithful" to their wives; they could only commit adultery with another man's wife, in which case the sin was against the other man. Some of this double standard was eased in the teachings of Jesus (Matthew 5:27-32), though, again, the different conceptions of marriage must be kept in view.
Theologically speaking, "adultery" is a pervasive metaphor for apostasy. The covenantal relationship between God and Israel is frequently portrayed as a marriage (see, for example, Hosea 1--3; Ezekiel 16). Israel's apostasy is consequently referred to as "playing the harlot" ("prostituting oneself," NRSV) or the like (Judges 2:17; 8:33; 1 Chronicles 5:25; Jeremiah 3:9-13; Hosea 4:1-19).
18 Neither shall you commit adultery.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org