Левит 18:8

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 (Lev 20:11; Deu 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 (Gen 49:4), and an accursed crime was committed, the punishment of which was death. At all events, it cannot be inferred from Lev 19:20-22 and Exo 21:9, as Knobel supposes, that a milder punishment was inflicted in this case.

Here is, I. A general law against all conformity to the corrupt usages of the heathen (Lev 18:1-5). II. Particular laws, 1. Against incest (Lev 18:6-18). 2. Against beastly lusts, and barbarous idolatries (Lev 18:19-23). III. The enforcement of these laws from the ruin of the Canaanites (Lev 18:24-30).