Jennifer, I had a conversation with a vanilla friend earlier that somewhat relates to this, I think it's worth sharing. She nearly had a fling with a prior romantic interest she hasn't seen in a while, who is now (recently) married. It was bothering her that she'd almost interfered with somebody else's relationship, and that the kind of interaction (being the mistress) wasn't something she was interested in. Frustration and guilt.

I casually threw out that it seemed like she may be a bit confused, about how she's feeling and how she's acting. There shouldn't be any indecision or difficulty acting, if you're sure of what you want, or don't want as the case may be. The situation isn't very difficult to manage on your end, if you really think about it. You can clearly explain to him that the relationship is inappropriate, or explain to HR that you can't supervise that employee for personal reasons.

I may be way out of line here, but the conflict seems to be personal for you. So perhaps I would suggest a bit of reflection, and decisiveness.