Paul explains why joining with a harlot is so serious. Some may have thought it was a passing physical act that meant nothing. Paul's question says otherwise: the man joined to a harlot becomes one body with her. His opening "What?" presses the point. Barnes says Paul illustrates the point from how marriage was formed in Genesis 2:24. Gill explains "joined" here as not in marriage but in an unlawful union, which still makes the two "one body" and "one flesh."

Paul answers from Scripture in the same verse: "for two, saith he, shall be one flesh," quoting Genesis 2:24. Then he sets the opposite union beside it: "But he that is joined unto the Lord is one spirit" (1 Corinthians 6:17). From this he draws the command, "Flee fornication" (1 Corinthians 6:18).

The Greek word for "joined" is kollaō, meaning to glue or stick, and so to unite closely. Paul uses the same word in 1 Corinthians 6:17 for the believer "joined unto the Lord." The quotation comes from Genesis 2:24: "a man... shall cleave unto his wife: and they shall be one flesh." Gill notes that the "he" in "saith he" may be Adam, Moses, God, or Scripture, and that Jarchi (Rashi, a medieval Jewish commentator) said it was the Holy Spirit. Jesus quoted the same verse about marriage in Matthew 19:5.

Paul takes God's design for marriage seriously. God made the joining of a man and woman to create a real oneness, "one flesh" (Genesis 2:24). That is why misusing it is not a small matter. The act carries a meaning God gave it, whether people intend that meaning or not.

But Paul's deepest point is the contrast in 1 Corinthians 6:17. There is a union even closer than one flesh: "he that is joined unto the Lord is one spirit." The believer already belongs in the closest bond of all, with Christ. Fornication would join what Christ has already claimed to a harlot (1 Corinthians 6:15).