COO is "an employee", but not really "just another employee". i.e. there is an entirely different level of responsibility (legal and moral/professional) for a corporate officer at that level, vs. a regular employee.
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.
