‘Flee fornication. Every sin that a man does is outside the body, but he who commits fornication sins against his own body.'
Thus picture now widens. What Paul is saying not only applies to consorting with a prostitute, it applies to all sexual misbehaviour. So there is only one thing to do with such desires of the flesh and that is, not to stand and fight them, but to flee (compare 2 Timothy 2:22). The man who would avoid the fornication or sexual misbehaviour which he is tempted to, must remove himself from the place of temptation and make his plans so that he is not put in that position again. And it is important to do so, for of all sins this is the only one that is actually a sin against the body itself, which has permanent effects within the body and the psyche, and which defiles as no other. And this the body which is one with Christ's body and a temple of the Holy Spirit. It is thus a direct sin against Christ to defile it by degrading contacts.
‘Every sin that a man does is outside the body, but he who commits fornication sins against his own body.' The context has stressed that the Christian has become one body with Christ's body. In the redemptive purposes of God he is one with Christ. When a man sins it reveals what is still within him, but it occurs outside the ‘body of Christ'. He does not make Christ and His body sin. But when a man commits a sexual misdemeanour his sin is actually affecting the whole body. He is uniting the body with a prostitute or fornicator. This is a heinous sin. He does not, of course, make Christ sin, but he produces an unacceptable situation in that part of him is united with Christ and part with a fornicator. He, as it were, tears apart the body of Christ.
Another way of looking at it is that, as with the previous verse Paul has to use a phrase that distinguishes one fact from another. In 1 Corinthians 6:17 he has had to temporarily drop for that purpose the picture of uniting with Christ's body, and speak of uniting in spirit, for that experience could in no way be paralleled with physical union with a prostitute.
Here he has to distinguish between sexual sin and all other sin. But Jesus had made clear that all sin comes from within, out of the heart of man (Mark 7:20-23). Sin results from contamination of the inner person. Paul is not denying that. He is not saying that sin is outside the heart of man, he is saying that while it comes from the heart of man its effects are outside the body. In other words it does not directly affect the physical body in its connection with the body of Christ in the way that sexual sin does. Sexual sin introduces sin into the man's body. All sin contaminates the heart, but it is effectually and clearly worked out outside the body. On the other hand sexual sin, he says, uniquely contaminates the body and all that that signifies. Its effects thus go even deeper. The man's body is contaminated and defiled. That defilement cannot, of course, enter Christ's body. Man can only be united with Christ once purified. He thus tears himself apart and robs Christ of what is His.