A manager fucking his subordinate is coercion at the very least due to the power imbalance.

This is not a consensual relationship for the same reason that sex with a "willing" person held at gunpoint isn't (even if there is no explicit verbal threat).

Whether it crosses a criminal threshold of sexual harassment or sexual assault , depends on the jurisdiction and cirumstance. In any case, it is not OK.

It is immaterial whether coercion of this sort is "not outright" rape.

Parent commenter's logic that justifies it because the CEO "has value" applies to "outright" rape as well.

This is what I have issue with.

You cannot just assert that a relationship is non-consensual without knowing anything about the relationship concerned.

No, because you didn't say anything about what the boss does in this situation.

The ethical thing for the boss to do is to decline having a relationship with a subordinate because of its potential to become coercive.

Same goes for a 13-year-old instigating a relationship with a 30-year-old.

What the 13 year old does is irrelevant. The 30 year old is the person with the power.

The coercive force here is the corporation that seeks to insert itself into its employees personal lives.

I cannot imagine avoiding a relationship in service to a corporation that has no hesitation to dispense with me whenever it is convenient. The relationship between employee and employer is entirely transactional and so has lesser priority than any interpersonal relationships that might develop.