The first command disallows a man from having sexual relations with his natural mother. In this instance, the nakedness of one’s mother is equated with the nakedness of one’s father. This reflects the original purpose of marriage, that two people become one flesh through the marriage bond (Genesis 2:24).
7 You shall not uncover the nakedness of your father, which is the nakedness of your mother; she is your mother, you shall not uncover her nakedness.