In the Old Testament there are two sexual sins denoted by the root words:
נָאַף
na'aph literally, this is the sin of a man having sex with another man's wife (or slave or concubine). It is usually translated "to commit adultery." This was essentially a sin against the woman's husband. The word is used figuratively of Israel or Jerusalem turning to another god, and thus the word refers to "worshipping idols."
זָנָה
zanah literally refers to a woman who "plays a whore" or "is a prostitute." Israelite women were not to engage in this profession (Leviticus 19:29; 21:9; Deuteronomy 23:18). It doesn't seem to be forbidden for other women to be prostitutes. When Israelite spies sneak into Jericho, they spend the night at the house of the prostitute, Rahab, who becomes a model of faithfulness (by lying!) (Joshua 2:1; 6:17, 22, 25). Usually, prostitutes were connected with pagan worship practices. A different word, literally, "dog," was used for temple male prostitutes (Deuteronomy 23:18). Figuratively, it referred to Israel or Jerusalem "prostituting" herself with other gods. They were being unfaithful to their God.
One of these sins centered on the man. He could not have sex with another man's wife (or slave or concubine). Otherwise, the woman's husband wouldn't know if the offspring was his and thus a rightful heir to his property.
The other sin centered on (Israelite) women. They could not be prostitutes, which probably implied being connected with pagan worship. Thus, idolatry. We also recognize that it would have been impossible to know who the father was should the prostitute become pregnant.
Both of the terms were used metaphorically to refer to Israel's unfaithfulness to God.
These two sins are expressed by two groups of words in Greek.
μοιχεύω moicheuō & μοιχάομαι moichaomai & μοιχεία moicheia= "sexual intercourse of a man with a married woman other than his wife," = "to commit adultery," "adultery"
πορνεύω porneuō = originally referred to "being a prostitute" or "making use of a prostitute." The connection with prostitution is still seen in NT times when the related noun πόρνη pornē always refers to "a prostitute" or "a whore." However, the verb and some other cognates in the NT era are expanding their meaning and can refer to any unsanctioned sexual intercourse. They are often translated, "to fornicate" or "fornication." (The Latin origin of these words referred to a "brothel," so the prostitution aspect is behind these words, too.)
Especially earlier, μοιχεία (adultery) and πορνεία (sexual immorality [related to prostitution]) were distinguished, but Sirach 23:23 includes adultery with sexual immorality. The distinction isn't so clear in NT times.
Because πορνεύω and related words were undergoing an expansion of meaning during NT times, we can't be certain what the authors meant when they used these words. At best, two of my Greek-English Lexicon for the New Testament note that the words refer to sexual immorality that is "often" or "frequently" related to prostitution.
