The situation was that some member of the Corinthian Church had committed adultery, and such an adultery was uncommon, even among sinners.  He had gone into his father’s wife, as Reuben, the first son of Jacob did (Gen. 35:22), and this incident was known in the Church (1Cor. 5:1).

It is possible to have such abominations in the Church today, where those who have been overcome by their passions (but are still in Church) desire or practically commit sin with their close relatives.  Another possibility is where unwatchful ‘pastors’ desire or even practically commit sin with members of theri flocks, or where unwatchful ‘brothers’ desire or even practically commit sin with so-called sisters, even though they be members of the body of Christ (as it were) (1Cor. 12:13 & 27).