If you are "going to be her boss" then you need to develop some managerial expertise. Firstly, get a copy of the actual policy. You'd be surprised how many people fret over something which isn't even a problem.
Now read that policy like a manager does. Is there any clause which says "people who are yet to be appointed boss are treated in this rules like they are already boss"? No, I didn't think so. So where exactly does your behavior to date cross the exact policy of your employer?
So, you're fretting about something which might be. Or mighty not be. Time to communicate with the other person and see what her desired future is. Maybe once with you was good enough to satisfy her sexual urge forever :-)
If you both want to continue then read the policy again, with an eye to whom exactly the policy applies to and what exactly are the exceptions.
You'd expect a pre-existing relationship to be an exception, so maybe you timed your make-out well.
What exactly makes a 'supervisor', what exactly makes 'staff'? Not everyone with "manager" on a name tag has supervisory responsibilities. And what are the boundaries: is being on disjoint shifts adequate? Is working in different divisions adequate?
What happens if you don't care. Arrested for child sex crime? Dismissal? Some sort of counselling love-in? Or nothing at all (aka "measures left to local management" whom you've made sweet)?
Let's not get too carried away with the drama of it all. The simplest way for work to tell you no is to ask them a question where the obvious answer is no. So why would you raise the issue at all: if asked say "as far as I am aware the relationship between myself and Ms X is consistent with the HR policies". It would be an idiot of a manager who would look into that further in the absence of a complaint (so, don't be lovely dovey at work thus encouraging Miss No Fun For Anyone to complain; don't discuss couple stuff at all at work).
Finally, some huge percentage of married people met at work. Despite corporations' best efforts at pretending they employ robots.