A Feeling We Always Chased (слэш)

"Okay, this is what is going to happen," Izuku began, "first, you're going to tell Shoto what happened."
Katsuki nodded and felt his gut clench and heart pound. He was absolutely panicking because he could not lose Deku. Some shitty part of Katsuki was hoping that, since Deku had already forgiven him for every other thing he'd done, that he could forgive this too. That hope was quickly squashed when Izuku spoke next.
"And second, you get a choice."
Katuski felt like his heart was going to leap out of his chest. Not even when they faced All For One together did he feel this hopeless and scared.
"The first option is that we break u-"
"No!" Katsuki shouted before Izuku was even allowed to finish. He wouldn't hear it or even entertain the idea. Without Deku he was nothing. "Anything but that."
"Then you've chosen the second option." Izuku walked towards the nightstand to grab his phone. "I get to have sex with anyone I want while you watch."
Or:
Katsuki cheats on Izuku and now has to watch his best friend fuck his boyfriend.