Treacherous

The one where you are Sebastian's girlfriend, but Chris can't get enough of you.

Due to the age gap between you and Sebastian, your boyfriend has a hard time feeling sexually attracted to you. In order to save your relationship, he invites Chris to have sex with you while he watches, hoping that the voyeurism will awaken his arousal and jealousy. Soon, he'll learn that inviting his best friend into his relationship may have just been the worst mistake he ever made, when Chris finds himself unable to let you go after his role is done.