You must not expose the nakedness of your father by having sexual relations with your mother. She is your mother; you must not have sexual relations with her.
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, or the nakedness of thy mother, shall thou not uncover,.... By uncovering a father's nakedness is not meant anything similar to what befell Noah, which Ham beheld with pleasure, and the other two sons of Noah studiously and with reverence to their father covered; nor any sodomitical practice of a son with his father; as Gersom interprets it; but the...
Lev 18:6-7 The laws against incest are introduced in Lev 18:6 with the general prohibition, descriptive of the nature of this sin, “None of you shall approach בּשׂרו אל־כּל־שׁאר to any flesh of his flesh, to uncover nakedness.” The difference between שׁאר flesh, and בּשׂר flesh, is involved in obscurity, as both words are used in connection with edible flesh (see the Lexicons).
Commenting on Leviticus 18:6-7