The way this manga is detached from reality from chapter to chapter crosses completely new boundaries. Even if you treat this title as something that shows "discovering life through sex and its various, not necessarily romantic sides", it's still disgusting and unreal, how these characters behave. I'm utterly disgusted by this chapter and with this manga overall. Either author's only experience in this subject comes from some hentais or... Or, I don't have any explanation.
Putting aside that this is all theoretically possible in reality: realism in fiction is overrated, and we all know it on some level. We're reading
comic books-- we may as well be watching cartoons. We'll readily put up with anything-- let alone "unrealistic" actions-- if the author can sell us well enough on it, or if it enables something else that we're fond of.
Myself, I'm not especially put off by the current shape of the work (though I don't find it groundbreaking, or even particularly good). On the other hand, you are. I don't object to the aversion you have, but it's certainly excessive to propose that his only experience on the subject of sex would come from hentai when 1) research material on this matter is far more accessible than hentai, and 2) he isn't obliged to express his personal experiences if he (and/or his editor) don't feel it conducive to the kind of work they want to produce.
Her character keeps being a mess. In the first chapter, she is super proactive about having sex with any random dude, but then she suddenly turns into a maiden. She is supposed to be a socially clueless girl that got taken advantage of, but she is not introduced in that way, at all.
...people who willingly sleep around are fully capable of being socially clueless.
She's bashful when talking about sex? Where was her bashfulness when she directly asked MC to plow her secondhand puss in the first chapter?
For starters: we're on the
sixth chapter, with each chapter painstakingly spelling out inflection points in these characters' developments like the ending segment of a
Dora the Explorer episode. Her bashfulness didn't exist in the first chapter because was acculturated into being the school bicycle in her bid for finding belonging (or, so she explains in the first chapter). She's actively building up a reservation about sex because she's since ceased to be the school bicycle and is furthermore in a relationship with someone who's not only unlike her previous partners, but also treats sex in a drastically different way than them-- which is on top of the fact that she only started being the school bicycle in the first place because she figured that was all someone like her could do to be accepted by others.
But that's what she stated she wanted from the start: a "normal love".
More critically, she didn't become bashful just because of a proposal of a foursome (which she presumably hasn't ever done, which would justify it by itself). She became bashful in a conversation about general intimacy. They're not just talking about having casual sex, wherein it's normal for the participants to minimize its significance to the level of playing video games-- they're emphasizing the aspect of "vulnerability" and how even simple intimacy serves to solve that.
Of course, the fact that she keeps trying to entice Kiyomi does indicate that she herself enjoys sex on some level-- I don't think her inviting him repeatedly is necessarily her old habits dying hard. It doesn't have to be anything-- she could have always had some interest in sex, or she could have an intrinsic interest in screwing with Kiyomi through her provocations. It may even be the author (however ineptly) using the two characters for a quick punchline that's meant to not be weighed equally with everything around it.
As an aside: I don't quite like Mayu. It's not impossible for me to buy her character as this libertine nerd (if nothing else, I've known more than one person like this), but she comes across more as a "character catalyst" given flesh rather than a character in her own right.
I'm pretty sure that impression is on account of her emotional range (particularly as it shows on her face) being far too narrow, on top of the fact that she currently exists nearly solely as "the girlfriend of a guy who himself is hardly characterized". And I don't think that be worth any indignation by itself, except I'm 50/50 on finding "libertine nerds" gross, and I do find Mayu gross. It's really-- it really is a personal thing, however heavily I weigh it.