I think Ahn thinks that his ex-bf sunbae does not really love him? Cos maybe the sunbae is jealous of Ahn before- like Ahn really excels in acting and in everything. And sunbae is frustrated that he cant be as good as Ahn. that probaby makes the sunbae loves him but at the same time, hates him too. Well that's just my opinion.
^this
The flashback isn't over yet. There's a little bit more of it in the next chapter and from what I understand (I could be wrong though) that's when the upsetting bit happens.
sorry meant to upvote
I’m a bit confused why what his ex said was so upsetting? It just seemed like he didn’t want to go into detail about their sex lives and dismissed the conversation since his partner wasn’t there.