Of course not. If a manager runs a tight ship and treats their team fairly, then there shouldn't be anything wrong with a relationship. If as a manager you can't properly separate your emotions and friendships from your management duties, you aren't going to make a good manager anyway. The manager who is going to unfairly promote the person they are seeing is also going to unfairly promote the worker who they are best friends with over the worker who best deserves the promotion.