A man is not to marry his father’s wife, so that he will not dishonor his father’s marriage bed.
30. A man shall not take his father’s wife. Since Moses does not here refer to any other kinds of incest, but speaks only of that with a step-mother, it is probable that, what he had more fully set forth before he here briefly recalled to the minds of the Israelites under a single head.
These laws relate to the seventh commandment, laying a restraint by laying a penalty upon those fleshly lusts which war against the soul. I. If a man, lusting after another woman, to get rid of his wife slander her and falsely accuse her, as not having the virginity she pretended to when he married her, upon the disproof of his slander he must be punished, Deu 22:13-19.
Commenting on Deuteronomy 22:13-30
A man shall not take his father's wife, nor discover his father's skirt. (l) He shall not lie with his stepmother, meaning by this all other variations forbidden in .