I would attach a note that says the standard confrontation script we reccomend
1. I want to save my marriage 2. Your affair is hurting our entire family
etc
I agree with pup, this was not an act of love, it was an act of control.. he's testing the waters.
You pointed out how emphatic you were that you did NOT want ANY secrets in the marriage and he goes and does this. He may be consciously or just subconcsiously challenging your "no secrets" theory.
I woulodn't change your position until you get some indication of an actual commitment to end his affair and go to family therapy. Any interaction he tries just attach the same note to it.