Canal knowledge of another person outside the marriage vows has no other purpose than the satisfaction of carnal lusts. Other sins such as overeating arise from outside the body, just as food comes from the outside of the body. But fornication is a sin against the body itself. The believer is a part of that redeemed body, or community, which is set up to glorify Christ. The sin of fornication contrasts with the true purpose of the body which is to reflect that glory.
Fornication is destructible of that purpose.
It is aimed solely at the satisfaction of the flesh and distracts the believer from realizing the essential purpose of the body. And do not present your members as instruments of unrighteousness to sin, but present yourselves to God as being alive form the dead, and your members as instruments of righteousness to God.