"ONE FLESH"
"Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." Genesis 2:24, referenced in Matthew 19:5,6, Mark 10:8, 1 Corinthians 6:16, Ephesians 5:31

A man is "one flesh" with EACH woman with whom he copulates, whether in marriage (wife) or in fornication (harlot). When a married man, who is therefore already "one flesh" with his wife, copulates with another woman, that does not then negate his being "one flesh" with the wife. This is evident by the fact that 1 Corinthians 6:16 reveals that a man can be "one flesh" even with an harlot. As even a married man, therefore, can become "one flesh" with an harlot, that proves that a married man can indeed be "one flesh" with more than one woman, without negating his being "one flesh" with his wife. As that is so even with a married man with an harlot, it is thus just as equally true regarding a man being "one flesh" with more than one wife. For further proof, the very next verse provides the context of the plural-to-one aspect, i.e., 1 Corinthians 6:17: "But he that is joined unto the Lord is one spirit." As EACH Christian is joined as "one spirit" with the Lord, that then demonstrates the context of the plural-to-one aspect. Namely, as EACH Christian is joined as "one spirit" with the Lord, so too may EACH woman be joined as "one flesh" with one man. Lastly, when the Lord Jesus, in Matthew 19:5,6 and Mark 10:8, was re-quoting that original "one flesh" verse of Genesis 2:24, He was only dealing with the issue of divorce, saying, "What therefore God hath joined together, let not man put asunder." (Matthew 19:6c-d.) That was opposing divorce of God-joined marriages, of what God Himself had joined together as "one flesh".

For context, it is exegetically important to note that the "one flesh" verse itself of Genesis 2:24, which the Lord Jesus was re-quoting, was written by Moses. And Moses married (was "one flesh" with) two wives: Zipporah (Exodus 2:16-21 and 18:1-6) and the Ethiopian woman (Numbers 12:1). The term, "one flesh", could not otherwise allegedly mean that a man could not be "one flesh" with more than one woman because three things did indeed happen. 1) Moses did marry two wives. 2) Moses did author such other verses as Exodus 21:10 and Deuteronomy 21:15. 3) Jesus Christ did not speak against Moses' being "one flesh" with two wives. Hence, the Scriptures reveal that Jesus and Moses knew what "one flesh" meant when Moses authored Genesis 2:24: a man may be "one flesh" with more than one woman.

From Biblical Polygamy
http://www.biblicalpolygamy.com/exegesis/one-flesh/