Mookyul's feelings weren't love but insecurity, loneliness and obligation since he was abandoned and the old man was the only one that had been there for him. Also he was never an uke, he said it in the sauna (the old man don't like to put any piercings or any ind of holes in him in more ways than 1) and in TC he told Jung Ewon the he and the president never had sex they way they do, which means it was as far as oral.
I agree with yessi...that mookyul's feeling for the president was more on the familial and obligation type of love since when he has nothing the president gave him everything and their sex are more on oral and touching only...
I seriously hated that Mookyul had somewhat feelings for the president. I had to admit, I hated how he was the bottom. Atleast, I knew some of his past. I liked the ending though :)