Fair point.
Well you can see it like fuckboi does everything to get slutkura so does fumi. And the writing is valid in this here. But than both are villains so idk.
The bad writing started way earlier. Reject fuckboi and he knows he has no chance with you. Reject blondie offer or talk with shinobu what he is saying to that.
Shinobu literally did nothing wrong and acted accordingly like a boyfriend would. Maybe he should say that he slept with fumi because he was drunk yet again he was drunk...
Also not to mention this chapter. Ive rarely seen a mangaka that put characters from earlier chapters in just for them being useless... airi really has talent in how bad her writing is.
If Grapara was well-written, everything logical and consistent, we wouldn't confuse about the plot's progression or the characters' thoughts and actions. LOL
But because the script is terrible and lacks coherence, characters even have self-contradictory behaviors, so we are here to guess about what is actually going on. LOL
In reality, the author doesn't seem to remember what she’s done or know what she's doing. So, there is no answer to many aspects of Grapara. Airi Katsura (sensei) will put anything into it without concerning of the consistency with the backstory or character traits. LOL