Eventhough she's a horrible woman, even willing to fuck a rapist, I doubt that cucking Takt be cause he asked her to do it is the reason why this shitshow is happening.
If he told her "Hey Shizuku I want your first time to be taken by Riku, while he initiates it and I want to watch." This bitch keeps saying that she "loves" Riku... Why would she even agree to this terrible condition then??
She's willing to emotionally destroy Riku, because that is what will happen once she fucks him and then forever stays with Takt, and she's didn't mind fucking a rapist. Those are not the actions of a woman that loves one specific man, those are the selfish actions of a woman that loves sex and herself.
"I love Riku" my ass... This is like revenge porn; she's out there to make Riku suffer.