I never understood how she thought that she’d get away with cheating in the first place. If she ever did end up having sex with the MC, he’d know immediately that she wasn’t a virgin. The lack of a hymen and her terrible acting skills would be a dead giveaway. Knowing her she’d probably admit it part way through (or call out the other guy’s name) because she’s a total idiot.
Honestly, Kondo is a piece of shit, but he wasn’t the one cheating. Miyuki, on the other hand, was knowingly trying to get away with it and would have lied about it until death just because it suited her better. I don’t know if she ever really cared about the MC in the first place TBH. If she ever thought his feelings were important, then she’d never have considered cheating in the first place.
I also don’t buy the “they’re young and make mistakes“ excuse at all. Mistakes are made by accident and without intent, but she made choices and knew they were wrong when she did it.