Commentators unanimously assume that the incestuous union in 1 Cor 5:1 is morally unacceptable even in the culture surrounding Corinthian Christians. They usually quote the words of classical moral philosophers or legal literature showing the moral outrage against this kind of sexual union in the pagan culture. In this paper, I will show that the teachings of Paul against incestuous union mentioned in chapter 5 are as counter-cultural as his teachings against eating idol food: the audience would regard the teachings of Paul as stances against their common sense. I will first investigate how popular poems and plays in the first century evaluate incestuous union and the extent of influence of these poems and plays to the society in general. It concludes that this specific kind of romantic relation of a man with his stepmother is well-received or even appreciated in the emerging social ethos at the first century of Greco-Roman world. It explains why the Corinthian Christians literally boasts of this union (5:2) instead of mourning for it. As a result, the meaning of the phrase “not even among Gentiles” (oude en tois ethnesin) in 1 Cor 5:1b will be reinterpreted in light of this new cultural understanding. This study confirms John Barclay's conclusion after comparing churches in Thessalonica and Corinth: the Corinthian church assimilated too well with the social values of the outside world. The double standard that Paul clarifies in 5:9-13 can then be best explained by his expectation of a counter-cultural Christian community that will execute judgment against immoral insiders.