It is always a mutual sin engaged in by two or more. Involving others in your own sin, You sin against them. If your convictions had been strong perhaps they would have been encouraged to do right. The sin, the shame, the wrong is shared by both. Temptation is not a sin fornication is.
Fornication is a sin against oneself.
The body of the believer belongs to God. He has been bought with a price and is to glorify God while in the flesh. All of this is clearly taught in this chapter. Fornication is generally committed. This leads one to believe that, since no one knows, or since the act was committed with a prostitute or call girl, that the sin goes unnoticed and the consequences are allayed.