How is sexual sin different from every other sin?
Sexual sin, especially fornication, differs because it is committed against one's own body, while every other sin is outside the body. For the believer, it also adulterates one's union with Christ, breaking the fidelity owed to Him, and violates God's holy design for marriage and extramarital relationships.
Les explains that Paul is stressing that the believer is now in union with Christ and must be as careful about fidelity to Him as a husband or wife should be to each other. Paul says every sin a man does is without, or outside, the body, but he that committeth fornication sinneth against his own body. Since the believer is now a totally different person, immoral activity adulterates his position with Christ. The believer has no business involving himself in extramarital relationships, because the family and marriage relationship is holy, sanctified, and ordained by God.
Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.