You must not have sexual relations with your father’s wife; it would dishonor your father.
These laws relate to the seventh commandment, and, no doubt, are obligatory on us under the gospel, for they are consonant to the very light and law of nature: one of the articles, that of a man's having his father's wife, the apostle speaks of as a sin not so much as named among the Gentiles, Co1 5:1.
Commenting on Leviticus 18:6-18
The nakedness of thy father's wife shalt thou not uncover,.... That is, who is indeed a man's father's wife, but not his own mother, but a stepmother or mother-in-law; or otherwise this law would coincide with the former; a man lying with such an one is accursed by the law, Deu 27:23; such an incestuous copulation was that of Reuben with Bilhah, and Absalom with...
The nakedness of thy father's wife shalt thou not uncover: it [is] thy father's nakedness. (d) Which is your stepmother.