Well, let’s believe that she was sooooo dumb that she didn’t understood that the other dude was hitting on her, and that even if she wasn’t flirting with him, ...
You have to really look at every single panel to fully understand just how bad she is here and that there is no room for benefit of the doubt at all. Zero.
You can NOT believe she is so dumb and didn't understand. On page 3 she instantly without any second thought goes "WAIT A MINUTE, IT IS NOT WHAT YOU THINK". She knows. Everything. What her secret date implies. What she did wrong. What she did with the other guy.
Saying that she can't learn about love with her literal lover and needs to secretly meet up with a boderline stranger and successfully understood love as a whole is a whole other kick in the gut... well, more like a nuke into his face. This is the first time I have read something this disrespectful and cruel in a romance manga. Her looking so content while doing it is rude as hell, lmao.
This also ties into the next point.
You can NOT type she wasn't flirting with him. She clearly is comfortable enough to meet up, secretly, to get incredibly drunk (enough to not make it home alone, he did escort her in the end, lmao) and go along with his flirtation pretty much always. Some of it clearly is unintended but she lets all physical contact happen (blushing once or twice) and encourages his verbal flirting with her answers.
She also actively shows that he has MASSIVE impact on her in a positive way. It is creepy as hell but he can actually go and say "Heh, I taught her what love is". It is weird.
She also is INCREDIBLY comfortable around him as she instantly pours out her heart and makes everything about her relationship, something private and without the knowledge of her boyfriend, public to a borderline stranger. That is CRAZY.
I looove age gap and mature romance. But there is no adult in this one (even if he behaves like one). I absolutely despise her.