I always highlight his good characteristics and what I liked about him. "You are a very accomplished and admirable man." I generally put in that I was attracted and that while we initially had chemistry, compatibility issues became more important. If pressed, I list the ways in which we do not match.

Recently (when pressed), I pointed out our obvious mismatch in fitness levels and how that was extremely important for me and unlikely to change for either of us. It still wasn't comfortable, but it was honest.