hi piglet
why did your first bf break up with you at the first place? Correct me if I'm wrong, but from I read in your post, K is your rebound, and you are still bitter about your first bf for breaking up. And you make it sounds as if you are doinghim a favour for taking him back..
And for your question, i really think that it's how mature your bf and your relationship between him is since you didn't say much there. You don't have to tell him what happened if you are uncomfortable, you don't even have to tell him anything. Personally, my bf ask me things with my ex as well.. and i know things that he done with his ex as well.. (we talk about almost everything.. including his fantasties) but these are cool with us as they are the past, and we know that they are history.