1_Corinthians 5:1:
a son had fornicated with his "father's wife".

This does NOT refer to the man's mother. Indeed, the term, "father's wife", is a very specific term.

Leviticus 18:8 refers to "father's wife" as specifically separate from "mother" in the previous verse of Leviticus 18:7. Note that the "nakedness" of a "mother" is referred to as her own "nakedness" while the "nakedness" of a "father's wife" is referred to as the FATHER's "nakedness".

This same differentiation is observed again in Deuteronomy 27:20,16.

In fact, what the fornicator had done as per 1_Corinthians 5:1 was the same sin as that of Jacob/Israel's firstborn son. Reuben had committed the identical sin with Jacob/Israel's wife, Bilhah, in Genesis 35:22. (Yes, Bilhah was Jacob's wife; see Genesis 37:2.) And for Reuben's act of "uncovering his father's nakedness" by fornicating with his "father's wife", Bilhah, Reuben lost his birthright as firstborn. 1_Chronicles 5:1 reveals that this was because Reuben had "defiled his father's bed".

Indeed, the reference to "father's wife" in 1_Corinthians 5:1 does reveal an actual polygamist identified in the New Testament, i.e., the father of the mentioned fornicator.