I'm from the UK and that strikes me as bizarre although I understand there is greater concern about these areas in the US. Unless you have hiring responsibility or they are on your direct reporting line I would see no need to report anything (even an ongoing relationship). That may change in the future if things are reorganized and a more direct business relationship is established.
Even then with a single date I wouldn't feel any need to report a single date unless they were reporting to me fairly directly and I was covering my ass.
A CEO is on everyone's reporting line and is also potentially a target so there is more of a reason for them to be careful and to report things.
None of this matters to me know as I'm married and am currently a one man company with no plans for staff growth.
The need is to prevent a situation as described in the post -- one where the party in a lower power position can charge that the party in a higher power position abused that power to force them into a relationship they did not consent to.
The more disclosure, the less likely that it would escalate. At the very least, there would be records that either party could use if something improper happened. For instance, there would now be written records if later on the person with more power did attempt to influence promotions that they should have explicitly not been allowed to be involved in that process.
I don't know why the UK would be different from the US from a legal standpoint, except perhaps just generally that the US is a more litigious society lately.
Even then with a single date I wouldn't feel any need to report a single date unless they were reporting to me fairly directly and I was covering my ass.
A CEO is on everyone's reporting line and is also potentially a target so there is more of a reason for them to be careful and to report things.
None of this matters to me know as I'm married and am currently a one man company with no plans for staff growth.