15 Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a harlot? Certainly not!
16 Or do you not know that he who is joined to a harlot is one body with her? For “the two,” He says, “shall become one flesh.”
17 But he who is joined to the Lord is one spirit with Him.
18 Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body.
Here [v.15], additional truth is added to that in verse 13. The body of the believer not only belongs to God and is for His use, but the body of the believer is united TO Him. This part of the personality of the individual believer is a member of the body of which Christ is the head.
In Romans 12:1 Paul begs us to present our bodies a living sacrifice, holy and acceptable unto God, which is our reasonable service. Ephesians 5:30 declares in unmistakeable language, “For we are members of His body, of His flesh and of His bones.” — Greene, p.224.
The quote in v.16 is from Genesis 2:24.
one flesh (v.16) — the sex act lives on morally in both partners, whether husband and wife or fornicators
one spirit (v.17) — spiritual union (John 14:20; 17:21-23)
flee (v.18) — sexual sins were widespread in Corinth