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 Tu ne découvriras point la nudité de ton père, ni la nudité de ta mère. C'est ta mère: tu ne découvriras point sa nudité.