Heesung might turn out to be not as much of a green/yellowish flag as he seems up to this point, but regarding Dan, he was very kind. He showed his affection to him, by being nice and buying him gifts and taking him out and treating him, even if it was a bit forcefully. He wanted to make sure (as shown in the spa scene) that Dan was not involved with Joo, before pursuing him, and what he said was that he wanted to "date" Dan, not just sc*** around with him, showing he really does like him. I know the plot surrounds Joo and Dan and not Heesung, but Dan would have probably been treated much better by him.