When a manager has a romantic relationship with a subordinate they could, theoretically, show that employee favortism and/or make their work life miserable. Now any promotion or raise or privilege the employee earns is under scrutiny. Did the Manager have any say in it? Was another employee dismissed out of hand for the position because they passed on the manager's advances? All of this is a problem when the romantic relationship is known and on the level, when its an affair it is VERY problematic. The employees involved in the affair may sue for retaliation due to some circumstance, or another employee may sue because they feel they were passed over for a promotion or position. Others might have suspected something and been made to feel their job was in danger if they asked questions.