Your hypothetical makes zero sense as your questions have simple yes/no answers.

> Even if it's consensual, what happens when she decides she wants to move on from the relationship?

Then they're no longer in a relationship and still have to follow HR/company policies.

>What about her peers that didn't receive promotions while she did?

Did they follow the company policy (e.g., hiring committee recusal) or not for the promotions?

>This is why these laws exist.

Did the company policies follow employment regulations when setting and enforcing their policy?