Hajin is deluding himself into thinking that it is still possible to be friends with the blond Guy. Once someone feels something more it is almost impossible to continue a sincere friendship. It gets worse because Geonwoo knows that Hajin's friend likes him, and that Hajin didn't have the courage to say that he's already going out with someone (even though they weren't really dating). He preferred to lie about a fetish than tell the truth, and once again he lied about what he was doing "so as not to worry" Geonwoo because he thought it was "not important" and nothing happened between him and his friend. And nothing really happened (at least on Hajin's part, the blond guy clearly wasn't there just as a friend) so he didn't need to lie about what happened that only raised more suspicions.
Hajin is deluding himself into thinking that it is still possible to be friends with the blond Guy. Once someone feels something more it is almost impossible to continue a sincere friendship. It gets worse because Geonwoo knows that Hajin's friend likes him, and that Hajin didn't have the courage to say that he's already going out with someone (even though they weren't really dating). He preferred to lie about a fetish than tell the truth, and once again he lied about what he was doing "so as not to worry" Geonwoo because he thought it was "not important" and nothing happened between him and his friend. And nothing really happened (at least on Hajin's part, the blond guy clearly wasn't there just as a friend) so he didn't need to lie about what happened that only raised more suspicions.