I'm not sure if I buy this completely. You see, a manager is human after all which means he/she is going to like some people more than others. There is always some kind of relationship with all the others, no way around it. Now obviously if the manager is only going to look at this relationship, his/her personal affection for others and not their work, and give some advantages over others based on that relationship, yes that is bad. Seeing that, and seeing an actual romatic relationship is just a more involved type of relationship, 'simply don't sleep with the people you manage' is obviously not enough. You should be able to put all or most personal things aside. As such I think these rules of no romantic relationships are a bit strange. Even if you follow that rule, but for the rest still manage people based on your personal relationship with them, it's still bad.