In the biblical tradition, when God’s people worship other gods, they are committing a kind of adultery—they are cheating on God (see Hosea for a good example of this kind of prophetic thinking). John does not have sexual activity in mind here. He is concerned with worship and fidelity to God, because worshiping God alone is one of his big concerns. The way he conveys this is to point out that if you are worshiping other gods or kings, including eating meat sacrificed to them, you are cheating on God. You are religiously promiscuous.
