If you go that route you can extrapolate to some absurd degree. Basically any relationship with a power imbalance becomes sexual harassment.

Yes, you can extrapolate, and our society does. This is why ministers aren't supposed to have relationships with their clergy, therapists are not supposed to have relationships with their clients, lawyers, doctors, etc. The list goes on and on.

Can become, yes. Depends a bit on what exactly the nature of the power imbalance is. Which is why there are usually strict rules about relationships with your reports and how to resolve the situation. Like reporting it to HR so the work positions can be shifted in a way to move positions so that there is no longer a reporting relationship between the two.

Exactly. At every place I've worked or consulted at, bosses dating subordinates is not permitted. I have seen multiple instances where people have to move teams/managers because they start a relationship.

These restrictions exist in most workplaces. Boss/subordinate relationships can result in extortion and bad behavior, regardless of which party starts it.

Congrats you just figured out divorce laws and alimony from first principles.

Wow you are mere inches from understanding the concept of sexual harassment. Keep going!