Eusan is what they call in full on denial mode. I've seen / read this in books. In the manga / manhwa world he's a tsundere. Hostile & standoff ish on the surface but he's not doing anything he doesn't want to do.
It's a very classic storyline!
Also, Ian was carrying the torch for 12 yrs, while Eunsan, even if ever had some sort of feelings for Ian in the past was hurt by him leaving and then got married to a woman (whom he either loved or really wanted to), and from his perspective, a former acquaintance he punched last time they met reappeares years later and is at first pushy and demanding, then supportive but clingy and then propositions him kinda out of the blue, then pushes for a situationship and confesses feelings... While planning to leave in 4 months again. Imo he's kinda confused, maybe not exactly to terms with his sexuality and doesn't want to get attached to someone who's left him once already...
Dunno if any of this rambling made any sense
you know what they say, beggars can't be choosers. We can see hints of Eunsan being in denial due to his low self confidence, but since Ian doesn't know this, he'll take whatever Eunsan gives him even if its a no strings attached fwb.
Does eunsan even like Ian? At this point Ian yearning is just pathetic eunsan is not returning even an inch of his affection