how would stone and his variants react to reader cheating on them?
Canon!Stone would be extremely hurt, he trusted you. And you cheated on him. He'd break up with you and probably wouldn't date someone else for a long time. Same goes for Girl Dad!Stone.
Grim Reaper!Stone thinks it's the ultimate betrayal. You made him consider himself as an actual person and not the weapon he thought he was and then you went and cheated on him. How could he have meaning and value if you threw his love away like it was a toy? So he closes himself back up, because if all he was to you was a toy, then surely that's all he ever will be.
Criminal!Stone wouldn't know what to do. He's hurt, but you're his mafia boss, his self-proclaimed deity. He can't just abandon you. So he attempts to brush it off, your importance is bigger than his and thus you should be allowed to cheat and hurt him. He's so down bad for you that he'll excuse it all. As long as he has the privilege of being in your presence.
Baker!Stone is hurt and will lick his wounds for months later, but he will hold firm and cut you out of his life. You want to throw away your guys' relationship? Fine with him, he has had therapy and he knows a healthy relationship. And he strives for one, so if you can't give it to him, he'll move onto someone else.
Feral!Stone is absolutely enraged by this because he's an arrogant piece of shit. What do you mean you cheated on him? No, no, that won't do. He's biting you and he might just kill you.
Serial Killer!Stone just becomes cold with you. You better hope you never commit a crime he finds egregious, because his love for you is tainted and if you commit an egregious crime, he'll hunt you down even with all of your romantic history.
Experiment!Stone's reaction is a lot like Grim Reaper!Stone's, in that he just closes himself off again. He thought he wasn't a monster, you taught him that he wasn't a monster. But clearly he was if you felt the need to cheat on him.
Vampire!Stone just scoffs because he'll hopefully be alive for such a long time and time does eventually heal emotional wounds. He tells himself that one day, he won't even remember you, much less the pain you brought him. So it's best he starts thinking of you as his past now.
Werewolf!Stone doesn't understand what he did wrong. Werewolves are meant to be family-oriented. He thought he'd expand his pack with you, grow old with you. And now everything's upside down. He's confused on how to feel.