The Greek word for flee
is the same word that is used in the Septuagint version of Genesis 39:12–13. Paul is thinking of Joseph, who literally fled when Potiphar’s wife tempted him to commit sexual sin.
Since in the sexual union two people become one (1 Corinthians 6:16), sexual immorality is different from other sins. Through sexual immorality, the integrity of one’s own body is violated. (Paul’s comment should be understood in light of 1 Corinthians 6:16, and not as a denial of the harm that other sins can do to the body or mind.)
18 Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.