Wow you guys... Koyuki wasn't trying to be a pain in the ass this is just who she is, she was trying to make things right for everyone, she wasn't expecting that reaction. Had she stayed quiet then laater they will surely offer her a drink too and then how far would you go to Not ruin the mood for everyone else? because even refusing when everyone is doing it or leaving makes things super awkward, is not something you can easily do alone in this kind of situations and SHE WAS ALONE there until Konatsu came, even if Konatsu's heart wasn't sure of her own motivation, I think she made the right call, I think Koyuki really needed to be "saved" from that situation.
Now they should try to talk... Konatsu has some really bad feelings going on inside her head that needs to be fixed and I hope that's what comes next.
Of course the author could've pulled off the "Looks like Koyuki didn't need me after all, she's happy with everyone" to make Konatsu feel more miserable than ever but considering things, what she did was more realistic, no way this teenagers would have some wholesome celebration and sings songs in the beach, it was a matter of time before Koyuki starts feeling awkward and distant if you suddenly throw her in a situation like that with no previous experience.
Also thank you very much for the chapter!!! it really feel long after the last with the cliffhanger, for a second I thought they wouldn't met and I was getting scared XP