You aren't piecing a marriage back together. You are divorced. You are not dating exclusively. It is not even clear that you are dating. Going in for the big R talk when only one of you is in that R is not advisable. It is pursuit, intrusive, presumptive, and suffocating in a big way.
That being said, I'm all for being direct and honest:
"Sorry XH, I've been fooling myself. This "just friends" or "casual dating" or whatever we are doing doesn't work for me. Instead, I need either more or less of an R. Given I will no longer be more into our R than you, it will have to be less of an R. For now, let's keep things strictly business. I sincerely wish you all the best. Time for me to find all the best for myself too. So, enough said. Ciao."
