1 Corinthians 6:16–18
16Or 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.”
17But he who is joined to the Lord is one spirit with Him.
18Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body.
The proposition I put forward is that Paul appears to be speaking to each of the house churches meeting in Corinth. They would have passed his letter around for each church to read.
In verses 16-18 he uses an illustration of what happens when a man commits fornication.
He who is joined to a harlot is one body with her. This is a principle from Creation; when a man is joined to a woman sexually, the two shall become one flesh.
The same principle applies to the man who turns from immorality is joined to the Lord, and realizes he is in Christ. In that case, he is one spirit with Him (the Lord).
Every sin that a man does is outside the body; stealing, lying, cursing, etc., comes from the body and is expressed outside of the body (the Greek word is ektos, out of). Sexual sins, however, go into the body (the Greek word is eis, into). While all sin is damaging, sexual sins seem to carry deeper inward personal damage.
A man should always choose to flee sexual immorality(KJV, fornication).
Paul will show in the following verses, the same thing will happen to the body of Christ, the temple of God, the local church.