Q. God told our first parents that when a man leaves home he should be united to only one wife (Genesis 2:24). This is surely the earliest indication that God intended monogamy to be the ideal and only estate of marriage. Further, you can only be "one flesh" with one person at a time. In polygamy you would become "two (or more) fleshes".
This famous "one flesh" passage is often cited by antagonists of polygamy so we ought to study it in a little more depth and just prevent doing any injustice to the writer's original meaning:
It baffles me why anti-polygamists have a problem with this passage because it is crystal clear. When a man and woman marry they become one - one in mind, heart, spirit and body...one in purpose, love, committment, and being. This is what marriage is - a uniting of two (or more) into one. If two women become united to one man, they also become one in mind, heart, spirit and body with him. They become "one flesh".
"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).
That this is true is made perfectly clear by the apostle Paul in a different context, for he says that "he who unites himself with a prostitute is one with her in body? For it is said, 'The two will become one flesh'" (1 Corinthians 6:16, NIV; cp. Ephesians 5:31). Thus if a married man unites himself with a prostitute he becomes "one flesh" with his wife and "one flesh" with the prostitute. We see that a man may technically, therefore, become "one flesh" with more than one woman.
What is Paul discussing? He is talking to the Corinthians about purity. He in effect says that Christians should be in a 'one spirit' relationship with Him in the same way that a husband should be in a "one flesh" relationship with his wife or wives (v.17). Christ has an allegorical polygamous relationship to His 'wives', the saints, being 'one spirit' with them. The "one flesh" relationship does not imply exclusive monogamy any more than the 'one spirit"' one does.
The choice here is between immorality or morality. A man may be one flesh with many women and be living in immorality (adultery, harlotry), or he may be one flesh with many women and be living a moral life (polygamous marriage).
There are some who maintain that "one flesh" means creating the product of two fleshes or bodies, namely a child. Though this is straining the meaning of the scripture it still does not alter the fact that a man may create many 'one fleshes', i.e. children.
A better argument from the anti-polygamous point-of-view (and we must be honest enough to consider all possibilities) is that one can only leave home 'once'. In other words, a man marrying polygamously would be required to "leave father and mother" many times.
If you look closely at Genesis 2:23-25 you will see that verse 24 is a comment - not on Adam and Eve (who had no physical parents) - but on marriage generally. All marriage is, if you like, a reverse drama of what happened in Eden: it is a new beginning and therefore requires a severance of sorts from the original family that nurtured those who are to be wed. Both the man and woman need to leave their original homes to start a new one. As a polygamist I 'left home' when I started my first marriage - all my wives have left their homes to enter my family. We have all left our parents, cleaved to one another, and become one flesh.
In the same way, as Christian/Messianic converts we have left the family of the world and cleaved to righteousness, and become one spirit with Christ. Yah'shua (Jesus) left His home in heaven in order to start the Christian/Messianic Family - the New Covenant - and has been cleaving with newly adopted sons and daughters in the Holy Spirit ever since. As the invisible Body of Christ (Messianic Community), we have begun to learn how to become one spirit with Him and with one another.
These spiritual patterns are to be found throughout the scriptures. As I have said on this homepage in many places, all marriage is, in fact, polygamy. There is no such thing as monogamy. The proof of the pudding is in the eating. Unlike the mongamist I have the advantage of having lived both monogamously as well as polygamously, so I speak not theoretically but from experience. I am one flesh with more than woman according to the righteousness of Elohim's Torah (God's Law) brought to completion in Christ just as I am one spirit with other believers purchased in the blood of the Lamb. The one bears witness of the other, and vice versa
 The Truth About Biblical Marriage, Chapter 11, One Flesh