And I will probably tell him about bp... because he just told me about a very personal medical problem... so... I will reciprocate his openness.

Confused, I sincerely wish my life were as simple and straightforward as that of your polyamorous friend. But, truth be told, it is not. It is asymmetrical: G knows about D but D does not know about G (I mean, he knows that G exists but that is the extent of it - he has no idea about ...). I am protecting him from this knowledge because I know that he would be hurt, and I do not want to be cruel towards him. People vary in how they treat such things.