Compare the case of Reuben, Genesis 49:3-4. See 1 Corinthians 5:1.

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, Deuteronomy 27:23; such an incestuous copulation was that of Reuben with Bilhah, and Absalom with his father's concubines or secondary wives, and such an incestuous marriage was that of the Corinthians, 1 Corinthians 5:1; and of Antiochus Soter, king of Syria, with Stratonice his mother-in-law (c): and even it was criminal to do this after a father's death, as Jarchi interprets it; and though she was only betrothed, and not married, and the father dead after such betrothing; as Gersom; nay, though she was divorced by the father, yet was not lawful for the son to have, no, not after his death:

it is thy father's nakedness; being espoused to him, and so one flesh with him; and the son and father being one flesh, such a mixture must be unlawful; and since then the nakedness of a mother-in-law is the father's, then surely that of an own mother's must be so likewise, which confirms a sense given of it in Leviticus 18:7, Cicero (d) exclaims against such marriages as incredible and unheard of, as instances of unbridled lust and singular impudence.

(c) Vid. Julian. in Misopogon, p. 72, &c. (d) Orat. 14. pro A. Cluentio Avito.

Intercourse with a father's wife, i.e., with a step-mother, is forbidden as uncovering the father's nakedness; since a father's wife stood in blood-relationship only to the son whose mother she was. But for the father's sake her nakedness was to be inaccessible to the son, and uncovering it was to be punished with death as incest (Leviticus 20:11; Deuteronomy 27:20). By the "father's wife" we are probably to understand not merely his full lawful wife, but his concubine also, since the father's bed was defiled in the latter case no less than in the former (Genesis 49:4), and an accursed crime was committed, the punishment of which was death. At all events, it cannot be inferred from Leviticus 19:20-22 and Exodus 21:9, as Knobel supposes, that a milder punishment was inflicted in this case.

The nakedness of thy father's {d} wife shalt thou not uncover: it is thy father's nakedness.

(d) Which is your stepmother.

18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services.