The vast majority of the Jewish community in Corinth had rejected the gospel (Acts 18:6). So the church was made up of mostly Gentiles who, of course, came from a culture that was not inclined to see sexual sin as unspiritual. Just the opposite. Most of the "religion" in Corinth involved temple prostitution and debauched sexual behavior.
That may explain somewhat why the Corinthian church would receive into their membership a man who was fornicating with his father's wife (1 Corinthians 5:1). Perhaps they thought they could connect with their culture better if they casually accepted the man's sin without flinching. In fact, it seems clear that some of the people in the Corinthian church did indeed wear extreme tolerance like a badge of honor. First Corinthians 5:2 says people in the Corinthian assembly were puffed up. They actually took some sort of perverse pride in their liberality towards such a grossly immoral act.
Not only was this guy's incest a supremely immoral and deeply shameful sin; it wasn't really impressing even the most immoral people in the Corinthian culture. Incest was a sin that even shocked the grossest pagans of Corinth (v. 1).
Paul wasn't gentle in his rebuke. He ordered the Corinthians to excommunicate the man (vv. 7, 13).
Notice: Paul wasn't impressed with how sophisticated and missional the Corinthians were. In fact (this can hardly be stressed enough) Paul never encouraged the Corinthians to blend into their culture by adopting an easygoing familiarity with or an extra-tolerant attitude toward the distinctive sins of that culture. On the contrary, he stressed the importance of avoiding the sins associated with Corinthian paganism.
No, I take it back. "Avoiding" is too mild in light of what Paul actually told them: "Flee from sexual immorality" (1 Corinthians 6:18).
But first he hammers them with several reasons why fornication is such an unholy, degrading, defiling sin:
It dishonors the purpose for which God made our bodies. (1 Cor 6:13)Now, put all this together. You want to know why fornication has always been regarded as a particularly heinous sin? Because it involves personal and direct transgressions against each Member of the Trinity. It debases and dishonors the body, which (v. 13) is "for the Lord." God created it for His purposes. To use it for any other purpose—especially a purpose as evil as an act of fornication—is a sin against God the Father. It's a sin against Christ as well (v. 15), because it takes our members, which are Christ's by union with Him, and joins them to a harlot, defiling our holy union with Christ. And it's a sin against the Holy Spirit (v. 19), because it desecrates the temple in which He dwells.
It defiles our spiritual union with Christ. (1 Cor 6:15-17)
Such sins of the body also desecrate the Temple of the Holy Spirit. (1 Cor 6:19)
And notice Paul's counsel to the Corinthians. He doesn't urge them to get into a recovery program for sexual addicts. He doesn't suggest that they get therapy. He just tells them to stop it.