It can be literal, describing sexual activity with temple prostitutes (as was happening in Numbers 25:1–18), and thus be a transgression in addition to eating food sacrificed to the idols. More probably, though, the reference is a description of what the Lord that thinks eating food sacrificed to idols amounts to; it’s being unfaithful to the Lord, the Bridegroom of the church. Repeatedly the Old Testament describes idolatry as immorality or adultery. In the context of Pergamum this teaching would advise that Christians need not avoid the city’s many temples to the gods and need not refuse to sacrifice to Caesar either; Christians can live comfortably in this world and be part of it. This is indeed the place “where Satan dwells”!
14 But I have a few things against you: you have some there who hold the teaching of Balaam, who taught Balak to put a stumbling block before the sons of Israel, so that they might eat food sacrificed to idols and practice sexual immorality.