OK, but I still don't see what HR could do in those cases with prior knowledge that they could not do with knowledge at the time an issue is raised.
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.
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.