I don't know where to begin. This chapter was so effective at what I assume its intentions were. I was absolutely miserable, but I think that's what the author intended.
Itosugi has thusfar positioned the relationship between Sora and exhibitionist-girl as ambiguous and the use of that ambiguity to put the reader into the protagonist's mindset for when things stop going as planned is outstanding. Explaining further, we don't know exactly what the party will entail, maybe the reader doesn't even think about what it will entail, all we know is that exhibitionist-girl is pairing with Sora and going. As readers, we don't know that this means in terms of romantic/sexual progression for these two, if any at all. We don't know that exhibitionist-girl is (basically) using Sora, the protagonist and de facto audience surrogate, not for romantic and emotional fulfillment, but for sex. Now when I say "using for sex" I don't mean directly like in the chapter of her introduction, because that's not precisely what happens. Instead, she leverages the trust of a friend(?) in Sora, to, without his knowledge or consent, use him as a bartering chip for sexual gratification. That's outstandingly messed up to do. Putting aside wether you find a giant, anonymous orgy as even something you'd be okay being included in, being pulled into that against your consent is really, really upsetting. Especially if the person who pulled you along, wanted expressly to see you see them. Exhibitionist-girl wanted to see the confusion, the unwanted desire, the guttural response, from Sora. And given that the two don't negotiate this event at all beforehand, we the audience, the reader, are right with Sora, hurt, confused, betrayed and maybe a little bit aroused against out better judgement as the emotional betrayal robs Sora, our protagonist and de facto audience/reader surrogate, of all agency.
It's. So. Perfect. To. Set. Up. Aki. As. The. Main. Heroine. The safety and security of home and family (sans a dose of Westermarck effect) becomes, more than before, that safe-zone where the confusing, judgmental outside world can't hurt you or use you. The ties of family (will presumably) become the necessary ingredient for forgiveness where other relationships would end, from the transgression of exhibitionist-girl's party that Sora will inevitably put on himself, because he betrayed Aki for another worman.
10/10, never do this to me again