What happens when a believer commits sexual immorality?
When a believer commits sexual immorality, he sins against his own body and adulterates his union with Christ. Because he is now a new person in union with Christ, such activity contradicts his position and violates the fidelity owed to the one to whom he has been joined.
Les explains that Paul is pressing the Corinthians about their past lifestyle and taking immoral activity for granted. Now, as a child of God, the believer is on totally different territory and is a totally different person. If a believer commits sexual immorality, he is adulterating his position with the one to whom he has been brought into union. Paul is bringing out clearly that the believer has no business involving himself in extramarital relationships. Fornication sins against one's own body, while other sins are outside the body. The family and marriage relationship is holy and sanctified, ordained by God that way.