1 Corinthians 5:1 teaches what fornication is: Leviticus 18.
Apostle Paul deplored the sexual relationship which involves a son and his stepmother, the woman (wife) of his father.
Paul should have referred to the relationship of the stepmother and the stepson as adultery as defined in Leviticus 20:10, but he did not because the sexual relation was not with a stranger or a neighbor but within the family.
And the man that committeth adultery with another man's wife, even he that committeth adultery with his neighbor's wife, the adulterer and the adulteress shall surely be put to death. Leviticus 20:10 ASV
(10) And Jesus lifted up himself, and said unto her, Woman, where are they? did no man condemn thee?
(11) And she said, No man, Lord. And Jesus said, Neither do I condemn thee: go thy way; from henceforth sin no more. John 8:10-11 ASV
Besides having sex with the woman of another man, there are other sexual relations which God Jesus forbids. The forbidden sexual relations are primarily listed in Leviticus chapter 18. The forbidden sexual relation of 1 Corinthians 5:1 which is called fornication (porneia) is sanctioned in Leviticus 18:8.
The nakedness of thy father's wife [woman] shalt thou not uncover: it is thy father's nakedness. Leviticus 18:8 ASV
And the man that lieth with his father's wife [woman] hath uncovered his father's nakedness: both of them shall surely be put to death; their blood shall be upon them. Leviticus 20:11 ASV
Therefore, fornication is the adoption of the forbidden sexual relationships in Leviticus 18 and elsewhere in Moses. The forbidden sexual relations in Leviticus 18 are called fornication not adultery which involves touching the woman of another man and which is the only cause for divorce.
And I say unto you, Whosoever shall put away his wife [woman], except for fornication, and shall marry another, committeth adultery: and he that marrieth her when she is put away committeth adultery. Matthew 19:9 ASV