"Then the man [Adam] said, "This at last is bone of my bones and flesh of my flesh; she shall be called Woman [Eve], because she was taken out of Man". Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh" (Genesis 2:23-24, RSV).
This, and parallel passages in Matthew 19:5-6, Ephesians 5:31, and Mark 10:8, are used to advance the argument that it is only possible for a man to be one flesh with one wife and that this excludes the possibility of plural marriage.
The Hebrew for "one flesh" is l'basar echad. As you will recall from Chapter 10 when we talked about the mystery of echad, this is a word that can variously mean 'one' or 'united' - it can also under certain circumstances be translated as 'first'. Basar can mean 'meat', or the whole 'body' or 'person'. The expression l'basar echad is a well known Hebrew euphemism or polite way of saying the 'pudenda' or 'external sexual organs'. In its simplest meaning, then, l'basar echad means to sexually unite viâ their sexual organs. Verse 24 may therefore be accurately rendered as:
""Therefore a man leaves his father and his mother and cleaves, clings or adheres to his wife, and they become united sexually".
Though there is no doubt that there is a mystic union between husband and wife whereby two become echad or one (which is the sense of Christ's teaching in Matthew 19:5-6 and Mark 10:8) on the physical level - which is what this verse is describing - here Adam is describing the fact that he is enjoying sexual union with Eve as a lawful spouse which marks the final break from the authority of their parents over their lives.
To summarise: 'one flesh' means to have sexual intercourse on the physical level, and to enjoy a mystic communion as echad or a 'oneness' on the spiritual level. Echad means two or more in one - it does not mean a single entity like "one tree" or "one cow" for which we have another Hebrew word, yachid. Does this being "one flesh" mean that a man can only have sexual union with one woman? Not according to Paul:
"He who unites himself with a prostitute is one with her in body. For it is said, 'The two will become one flesh'" (1 Cor.6:16, NIV; cp. Eph.5:31)
Now we all know that uniting (becoming echad) with a prostitute is against Yahweh's commandments and sinful because this is not a marriage with covenants. Nevertheless it lays to rest the lie that a man cannot be "one flesh" with more than one woman at a time. A married man with one or more wives can lawfully be "one flesh" with them all and attain a mystic echad union, as Christ enjoys with His uniplural Bride (us), and he can be unlawfully sexually united or "one flesh" with a prostitute and, by extension, with another man's wife ... this time adulterously. Or he can be "one flesh" with an unmarried woman (fornication) and thus be obliged to marry her, whether he is already married or not (Ex.22:16).
Christians are supposed to be in a "one spirit" echad relationship with Christ as His Bride (1 Cor.6:17) in the same way one or more women are supposed to be in a "one flesh" echad relationship with her/their husband. That is all being "one flesh" means.
Finally, let us not forget that the man who penned the verse in Genesis - Moses - was himself plurally married with two wives - Zipporah the Midianite and an Ethiopian woman (Ex.2:16-21; Num.12:1). Would Yahweh get His prophet to write a Scripture limiting a man to only one wife and then punish that prophet's sister (Miriam) and brother (Aaron) for complaining that he had two wives? Shouldn't it have been the other way round? Shouldn't Yahweh have thanked Miriam and Aaron instead of chastising them, and then ordered Moses to put aside his second wife?
"Then Miriam and Aaron spoke against Moses because of the Ethiopian woman whom he had married; for he had married an Ethiopian woman ... So the anger of Yahweh was aroused against them, and He departed. And when the cloud departed from above the tabernacle, suddenly Miriam became leprous, as white as snow. Then Aaron turned toward Miriam, and there she was, a leper" (Num.12:1-2, 9-10, NKJV).
Well, Yahweh didn't thank Miriam but punished her. He vindicated His prophet. When people make 'one flesh' out to be a monogamy-only passage they are accusing Yahweh of hypocrisy and duplicity! Shame on them!