The Greek word 'porneia' is a certain sexual activity. Why is it used to describe non physical acts such as Revelation 2:21, 14:8, 18:3, etc?
Eph 5:30 For we are members of his body, of his flesh, and of his bones.
Eph 5:31 For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.
Eph 5:32
This is a great mystery: but I speak concerning Christ and the church.
God views consorting with any false religion or false doctrines as spiritual adultery...
Rev 17:3 So he carried me away in the spirit into the wilderness: and I saw a woman sit upon a scarlet coloured beast, full of names of blasphemy, having seven heads and ten horns.
Rev 17:4 And the woman was arrayed in purple and scarlet colour, and decked with gold and precious stones and pearls, having a golden cup in her hand full of abominations and filthiness of her fornication:
Rev 17:5 And upon her forehead was a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH.
This is symbolic of a great false church.
1Co 6:15 Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make them the members of an harlot? God forbid.
1Co 6:16 What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh.
1Co 6:17 But he that is joined unto the Lord is one spirit.
1Co 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
1Co 6:19 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?
1Co 6:20 For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.
Notice verse 18, the word fornication there comes from the Greek...
G4202
πορνεία
porneia
por-ni'-ah
From G4203; harlotry (including adultery and incest);
figuratively idolatry: - fornication.
Paul was not only referring to physical sexual sin here, he was referring to idolatry.