So basically, he isn't your boyfriend (yet) but you two are close friends, so you where at his house, just chatting, and giyu is your boyfriend, and you got a notification that mitsuri had posted something on her Snapchat, so you looked to see her and giyu kissing, meaning both you and obanai had gotten cheated on, so you and him teamed up, so when mitsuri would call obanai, you would pick up instead of him, and obanai would make pleasured noises and clap while you talked in a pleasured tone, knowing both giyu and mitsuri could hear.