no they did not, and u mis understand it, hangun didnt replied when won asked him the "I... did" answer was also won answer to his own questioned, its like he confessed to Hangun that he slept with other people in the 8 yrs that they are not in touch and Hangun said he wont mind that because he expect all that when he left won, you can read it again and understand that its all won's monologue.

Someone who read the novel, did hangun slept with someone else during those eight years? I couldn't understand that part. I'm kinda disappointed Won did though.. and same would go if hangun also did :(