Eating of food and drinking of liquids for the slaking of thirst is not sinful. Excess in eating and drinking is a sin. This intemperance does harm to the body physically. The nature of the sin of fornication is different.
Fornication is any form of sexual unchastity.
Any act of fornication is a sin in itself. 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.