I see some people confused about Yoshino's motives dealing with any of this, especially Kirishima.
You all have to remember that she was raised in and around this world of yakuza. She's basically a yakuza princess or something right? She has not dealt with the gory bits directly but she's not completely clueless to her position and her place in it all. She's also not stupid. It's not like she has much of a choice but to "go with the flow". Her options are deal with her engagement/bodyguard-ship/stalker-ship to Kirishima or some kind of violence between the families that will surely fall back on her old man in the midst of different yakuza regions fighting. She too, feels she has a duty obviously.
Remember the extra chapter page where it says how she reacts when a man tells her to be quiet and she just shuts down? Think about it for a second who Yoshino is.
Edit: She doesn't care about his cheating because they're not in a real relationship at this point. Come on guys, she said this straight up in the manga. I'm pretty sure he's not considering it a real relationship either. Yeah, he weirdly declares his love but do you guys think he actually...loves her right now?
I also am halfway rooting for her childhood friend but he ain't on the cover art lmao. Whatever budding romance is intended for Y×K is going to happen(obviously) so we might as well just try to enjoy Kirishima basically learning how to be a decent human being (probably).
If not I will eat my foot and have mad respect for the author to have YxK just be a friendship instead of romance...but...I doubt it will be like that.