@JohnDB There were probably plenty of Romans who lived something like that, but there were probably those who had one wife, actually loved their wives. And there were probably those that hid their girlfriends. Some Roman men lived on the daily dole. They weren't all rich Playboys. And if they weren't in the upper classes divorcing and swapping out wives as a political maneuver.
Ephesus was in a more Greek-influenced area in Anatolia. The core group of a lot of these churches, the group they started with, would have been made up of Jews, proselytes and Gentile God-fearers--uncircumcised Gentiles who feared the God of Israel who had been listening to Torah. The Jews probably had better sexual morality than their pagan neighbors. In Ephesus, the church had grown among Gentile populations, also.
Roman law about who was recognized as a wife applied to Romans. Rome allowed the Jews to have their own customs and legal system, a system headed by the high priest for Jews throughout the empire.
My understanding is that a concubine for Jews is different from what concubines were for Romans. A Jewish concubine is a wife whose status was formerly slave before getting married.... typically to her master or master's son.
Btw, in what letters do you see Paul referring to consorts as full wives?