Dense? Or is she totally in love and thinks blonde girl is the dense one?
You know, I really would have liked the story to go there. To examine in a little more depth exactly who is the dense one here.
Because, for all we know, Shino is aware of Rumika's feelings and has been waiting like forever for her to make the first move. And, in the end, realizing her approach wasn't working, she decided to go for a more aggressive tactic and hit Rumika with a double whammy:
Pretend there's a cool guy macking on her. (Make Rumika jealous.)
Ask for kissing lessons.
Shino: "And if THIS doesn't work either, I'll have to resort to the old trick zen masters use to bring their disciples to enlightenment and smack her on the head with a big stick."
Ah, the classic NTR setup. Girl gets new boyfriend! But she wants to make sure her technique is up to par so she can be a better girlfriend. She loves him very much and he deserves the best after all. But when they finally get to it, the girl's "practice" partner is actually better than her boyfriend. And it's a slippery slope from there.