While class is definitely a factor for Jihwa, I suspect it has more to do with the fact Seungho is showing Nakyum special attention. He could deal with all the other random men because none of them meant anything to Seungho and Jihwa has remained his 'number one'.
But shortly after Nakyum arrives that all changes. Suddenly Seungho stops particating in orgies with the other nobles and rumours of a him having a finance or mistress start. Despite having a seemingly special relationship Seungho casts Jihwa aside the instant he complains about it.
I do feel bad for Jihwa but the way he is venting his hurt and frustration is not healthy. Hurting Nakyum is never going to bring Seungho back to him but he just can't see it
We're afforded a peek into Seungho's and Jihwa's past together. Apparently, they were both discovered to be gay from an early age. It is implied, however, that Jihwa overall fared better than Seungho for his sexual orientation. Whatever happened to Seungho, he developed hypersexual tendencies as a result and made a name for himself as a sexual fiend. It's one thing to sleep around discreetly with other men, but Seungho participated in orgies and maybe exhibitionist displays? (Seungho thighfcking Nakyum in the back of a tailor's shop may not have been his first time doing something of the sort -- considering his reputation, it's possible he has done more explicit things in a public setting before).
Per his report, Jihwa has been holding a candle for Seungho since they were both children. It was a point of pride for Jihwa early on in the story for knowing how best to turn on and please Seungho in bed. Jihwa feels like since he's known Seungho the longest, he knows Seungho the best. He's apparently been waiting for Seungho to one day settle down and see him as more than a fckbuddy/fwb. But while Jihwa could handle Seungho sleeping around with other men of nobility, he drew a hard line when Seungho started looking at Nakyum as a potential partner. The classism is strong among these men of nobility (Seungho, Jihwa, Min) -- enough that Jihwa and Min don't blink twice at planning to kill Nakyum for taking Seungho's attention and expecting Seungho to just return to them once Nakyum is dead. I'd feel bad for Jihwa, but his complete disregard for human life (even if it's a life that is worth "less" than his since he's a noble and Nakyum is of the lowest class in society) extinguished any pity I may have had for him -- especially since he wants to off Nakyum because he's jealous and hurt that Seungho won't reciprocate his feelings ( ̄ε(# ̄)Σ