Leviticus 18:8

The nakedness of thy father’s wife shalt thou not uncover: it is thy father’s nakedness.

Sexual relations with a father's wife are forbidden, and to expose her is to expose what belongs to the father himself, violating his own intimate boundary. This prohibition stands among the Lord's commands given to guard the moral purity of His people. Any sexual liberties and familiar conduct that cross this line are crimes and sins.