It was a blackmail. And if he was asleep, as much as "cheating" is considered, this situation is no different than r*pe.It's getting infuriating to read chapters now with so much stupidity
Rika tried to use misato to have sex with him,i think not to blackmail him but break him mentally.Think about it if sosuke have sex with anyone else then he will be the same as those cheaters then he won't be able to blame rika or anyone else but himself.
Since rika's misato plan failed and she even lost leverage against misato,she invited sosuke to orgy party where he will have to compromise his morals inevitably.
He will have to go with a female and there he either have sex with same partner or different females.In both case he is doomed.
I hope they don't make a contest or a tournament for couples there and the final couple will be able to meet the boss or rika and get some prize or something.That's probably where we will see rika's whore past and how she is the best earner of that place.
Iwabuchi isn't the mastermind for sure,he is just in love with her like sousuke.She doesn't show the same genuine happiness to him as she shows to sosuke in her own twisted way that's why he got annoyed.
There was also a precise control in this case: Rika had full hold of Misato, but on another instances, she used the ultrasound to scare Akiba. If both could've been used for the same goal, why not just let the young sensei have her way, then?
And if the video was indeed made and sent to her, although guilty trapping him, who knows what her reaction would be? How would she proceed? We don't know what this "Greatest love" of her is, and considering how much messed up she is, who knows what that means for Him?
Maybe she wants complete submission, and if Akiba had her way, Sousuke would fall out of love for Rika and compromise her whole plan?
Maybe Iwabuchi knowing of Sousuke's ideals of "cheaters must be punished" is an indicative that Rika was the one who knew of this and wants Sousuke to destroy this logic by understanding there's true love between two people who cheat and enjoy cheating( like an open relationship)?
Maybe Rika wants to steadily, but surely, control the entire process of Sousuke breaking, so she sees him turning into a despicable person, to mold him to her tastes?
Maybe her tastes align with her wanting to break and mold him into a man worthy of her, one who could get a hold of her, subjugate her, and show love only to her, even if he has sex with other people?
Maybe she wants Sousuke to kill himself to prove his love for her? Or maybe break him enough to propose a lover's suicide?
In the end, there's a lot of potential routes for this love of her. The problematic of the missing piece is literally,
Why she thinks of this "Greatest love" -> Why she breaks people -> Why she needs Sousuke.
I remember this guy from Fate, Beryl Gut. Is way of showing love was to hurt the person he loved, phisically and mentally, because that's how he was teached what love was. Some people love by giving their existence to another treat and mistreat them as wish. So who know what's in her head?