The proper HR response is to remove the higher ranking officer from an "evaluatory role" and put someone else in charge of determining future promotions, demotions, bonuses, team role, etc.
Let's imagine a horribly messy situation. A professor and a graduate student that works in her lab want to become intimate. That's perfectly fine, so long as the professor gives up the ability to determine if they can graduate, dictate what classes they take, dictate pay, etc. Even cleaner because there is a record that the professor is to not be asked to evaluate performance in the future, so there's less likelihood that the other professor could be influenced.
You at some point do have to assume that people are trying to do things correctly, of course. But the more people there are who are aware of the situation the less likely it is that a conspiracy to cover up abuse of power will occur.
You still have a sexual relationship between two people in the same organization with a big power differential between them.
Shouldn't the proper HR response just be "you need to absolutely not do that?" What can HR do beyond "keeping an eye on things?"