1 Corinthians 6:16
What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh.
Cross-reference
1 Corinthians 6:9 warns the sexually immoral will not inherit God’s kingdom — adding the consequence to the union-with-prostitute argument.
1 Corinthians 6:19 calls the body the Holy Spirit’s temple — directly extending the argument against joining to a prostitute.
1 Corinthians 3:16 declares believers are God’s temple — grounding 1 Cor 6:16’s argument that sexual immorality defiles the body.
Genesis 2:24 is the exact verse Paul quotes — the creation ordinance that 'the two become one flesh'.
Matthew 19:5 records Jesus quoting the same Genesis 2:24 on marriage — Paul applies it to a sinful union.
Matthew 19:6 continues Jesus' teaching on marriage as a divine union — Paul uses 'one flesh' for illicit union.
Mark 10:8 also records Jesus quoting Genesis 2:24 — Paul applies the same phrase to prostitution.
Ephesians 5:31 quotes the same verse positively for marriage — Paul here uses it to warn against prostitution.