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Mind control...
Literally 90% of such out of character actions requires some degree of mental alternations or manipulations to be believable.
Though often it is peer/social pressure's, or things like alcohol & weed.
Even most(okay, just some, +/-33%) porn, has that limitation... When it pays attention to plot, at all.
Erotica with established characters, or even supposedly average/normal "real life" style characters, is less believable if there is no driving force behind the erotic actions.
I mean, it's not like everyone is getting sex on even a weekly basis unless they're married.(excluding outliers who are satisfied with only meaningless sex, and workers in the sex Industry.)
And a lot of erotica suffers from a lack of passing the Fridge Logic test. As it glosses over the problems that would happen if its narrative happened in reality... Or even just in the reality of the story.
Which is why, Akane must have some form of mental manipulation going on... Unless it's a social norm in the world she's in.
Possibly extreme hormones, or something like a this is normal, action blindness field. Where she's not actually aware that there is something odd with what she's doing or her body.
Given that she's smiling, probably not something like blackmail or other coercion.