God pretty much tells Moses how to understand what He means by "whoredom" as He speaks to Moses about what Israel would do before He calls Moses home in Deteronomy 31:14-18:
14 Then the Lord said to Moses, “Behold, the time for you to die is near; call Joshua, and present yourselves at the tent of meeting, that I may commission him.” So Moses and Joshua went and presented themselves at the tent of meeting.
15 The Lord appeared in the tent in a pillar of cloud, and the pillar of cloud stood at the doorway of the tent.
16 The Lord said to Moses, “Behold, you are about to lie down with your fathers; and this people will arise and play the harlot with the strange gods of the land, into the midst of which they are going, and will forsake Me and break My covenant which I have made with them.
17 Then My anger will be kindled against them in that day, and I will forsake them and hide My face from them, and they will be consumed, and many evils and troubles will come upon them; so that they will say in that day, ‘Is it not because our God is not among us that these evils have come upon us?’
18 But I will surely hide My face in that day because of all the evil which they will do, for they will turn to other gods.
Once again...when you let scripture interpret itself, you will be on safer ground. As I said before...follow this up with these passages to understand who the Harlot is in Revelation:
Isaiah 1:21-23
*Jeremiah 2, 3
*Ezekiel 16
*Hosea
*Amos 7:14-17
The harlotry is spiritual...and it's the failure of Israel/Judah to be faithful to God, whom they covenanted with on Mt Sinai at Exodus 24:3-8.
As you read Revelation it is filled with prophecy that comes from the OT prophets...and the adultery is that of Israel who was Gods covenant people. Revelation 17:4 is speaking of the things of the Old Covenant tabernacle/Temple of Israel, which was the symbol of Gods presence among them:
4 The woman was clothed in purple and scarlet, and adorned with gold and precious stones and pearls, having in her hand a gold cup full of abominations and of the unclean things of her immorality,
This is not hard!!! Israel was the harlot, and God judged her by destroying her temple and wiping them off the land in 70AD...using Rome as His instrument of judgement!