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 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.
And because a reporting relationship between sexual partners is a bad idea.
"I think we need to let X go" could take on a whole new context if the speaker was just dumped by X.
For example, if I have just been fired by the manager who I just dumped, I go to HR and have them resolve the situation then and explain the conflict of interest given that we were in a relationship. The manager is given a talking to, I keep my job and the situation is resolved. Please explain how this situation plays out differently under your rules.
I would counsel against but not fire engineers who were dating each other within a group (and, in a larger organization, if it's outside of a specific group, wouldn't be a problem. I think a good manager would give them each an opportunity to change roles so they're not directly working together.)
In the intermediate case of a manager dating a direct report, the problem is with the manager, not the direct report.
COO/founders/etc. don't get any slack for things like that, though, even if they are not directly managing the other relationship party.
And if you read that, you're still asking why HR has any right to know if their executives are in a possible enanglement with a subordinate?
OK, maybe you have a reflexive hate of HR. don't fixate on them. Instead, pretend you were in the shoes of Square's founders or major investors, and you're reading a lawsuit that accuses your COO of sexual harassment, the kind of sexual harassment allegation that can only come from a COO who broke standard operating procedure (I.e. not informing HR of this)
Would you think the investors and partners are more angrier or less angrier than you right now?
A person no matter what level he is at who has a genuine consensual relationship with another employee should not cause the company any issues which HR would need to resolve so I still don't understand the need to declare all such relationships to them.
For what it's worth, in the UK it's perfectly OK to have a secret office romance and such things are not required to be declared. It could be that our cultural differences are the main reason for this difference in opinion.