A man cannot be one flesh with more than one woman!
Genesis 2:24 "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." Matthew 19:5-6, Ephesians 5:31, Mark 10:8
1 Corinthians 6:15-17 "Know ye not that your bodies are the members of Christ? Shall I then take the members of Christ, and make them the members of an harlot? God forbid. What? Know ye not that he which is joined to an harlot is one body? For two, saith He, shall be one flesh. But he that is joined unto the Lord is one spirit."
A husband and wife have a "one flesh" relationship, which is very important, being the fundamental essence of marriage, and a physical reflection of the "one spirit" relationship between Christ and the church (1 Corinthians 6:15-17, Ephesians 5:22-33).
Can a man only have a single "one flesh" relationship with just one woman?
1 Corinthians 6:16 states clearly that if a man has sex with a prostitute, he becomes one flesh with the prostitute. Clearly, if a single man has sex with a prostitute he becomes one flesh with her. But was this verse written only to single men, or to married men also? Nowhere is this limited to single men. So if a married man has sex with a prostitute, he must become one flesh with her. But he is already one flesh with his wife. So now he is "one flesh" with one woman (his wife), and also "one flesh" with another woman (the prostitute).
So a man can be "one flesh" with more than one woman.
To look at this from another angle, if "one flesh" is a parallel of the "one spirit" relationship between Christ and the church, what are the spiritual implications of this?
1 Corinthians 6:17 states that "he that is joined unto the Lord is one spirit". Every individual who accepts Christ as Lord becomes one spirit with Christ. Each individual believer has a "one spirit" relationship with Christ. It doesn't matter how many other people become Christians, each one receives this same intimate "one spirit" relationship with Christ. Ephesians 5:32 on the other hand states that "Christ and the church" are joined in this way, and that passage is often used to imply that the church is the one monogamous bride of Christ. But the Greek word for church, "ekklesia", means "a gathering or assembly of people" - it is not a word meaning a single individual, but rather a group. So Ephesians 5:32 also states, in the Greek, that Christ is one spirit with the many individual people who follow Him.
Every Christian has a "one spirit" relationship with Christ. However many others join the church in future, each has their own individual "one spirit" relationship with Him.
Every wife has a "one flesh" relationship with her husband. However many others join the family in future, each has their own individual "one flesh" relationship with him.