Yes you do. But in case you didn't really experience it, I'll explain it to you. It's not easy to forget someone you've liked for a long time. You may feel like you are completely over them if u don't see them for a long time after the first fall out, and you are starting to feel something for someone else (which is the case of our protagonist here). Then you see that person again, you feel your world crumbling and you may even question your feelings for that new person, especially more so if u just started having a little crush on them. While I do think it's wrong to full-on invest on a new relationship when you are not completely over someone, it's also foolish to think reality is always black or white. Sometimes it's gray too. I understand the fact that the protagonist is shocked by hearing the confession from the senior, but he also states he has no intention of ever going out with her, and he is ashamed of his feelings. And also, fix your reading comprehension, because he never says Hae In was going to be an option, since he openly states he is in love with her and has no intention of going out with the senior. Unlike you, I speak from experience.
I get what u’re saying, like yeah it’s not easy to forget someone u liked for a long time. It’s normal that ml got confused when his old crush came back. But still, from fl's side, imagine just kissing someone and then hearing he was kinda happy about his old crush confessing??? Even if he says he doesn’t want her anymore, it still makes his feelings look less certain. I’m not denying that he likes fl, that’s obvious. But the fact that he’s still confused makes it feel like she’s not the priority, that’s why i said it seemed like she was being treated as an option.
and honestly, by that logic, i guess i can’t comment on murder mysteries unless i’ve killed someone, no? u don’t need to go through something to understand or have an opinion on it and lastly, different interpretation doesn’t mean poor comprehension. people can read the same scene differently, and even if i haven’t been through the exact same thing, my opinion is still valid. u have ur own take, and i have mine :))

Bro the guy starting to piss me off, like my girl don't deserve to be an option !!