Saying "she can't consent because he could harm her" is like saying I can't deny giving my neighbor some milk because if I upset him he has the power to break all of the windows on my house. Since when do we focus on these hypotheticals? Since when do we always assume malice? We trust that people are good, prosecute the bad ones (both breaking windows and firing over sexual denial are illegal), and allow neighbors to ask for milk because 99% of the time no windows get broken in the event we say no.
Sure, the analogy might be a bit contrived. But I think it still does a good job of demonstrating how ridiculous this line of reasoning when applied outside of the sexual harassment thought bubble.
Your next question is a separate issue and one that does not deal with harassment. It deals with favoritism in general. Whether it's favoritism for sexual reasons or because you and your boss both play on the same proam bowling league doesn't seem particularly relevant to me.
I’m glad my point is convincing enough to move on to the next points.
Likewise, it doesn't matter if dating a subordinate is illegal. It's wrong and should be against corporate policy. Corporate policies allow us to address lots of antisocial behavior that doesn't rise to the level of illegality.
Inappropriate relationships with subordinates are, well, "inappropriate", and can be sexual harassment when they are persistent and unwanted, or involve any sort of quid-pro-quo. That, however is not a crime, but only a violation of (civil) employment law.
And that doesn't change the fact that obvious nonconsensual sexual advances are not the same thing as dating at work in a relationship with a superior. Inappropriate as it may be, it certainly isn't illegal, now is it?