Izuocha

Izuku would cry a lot and wouldn't talk to anyone for a long time

Ochako would just wonder if she wasn't good enough for him and start doubting and feeling terrible about herself

Todomomo

Shoto wouldn't know what to do. She was the first person he ever loved and her cheating hurt him more than anything ever

Shoto wouldn't cheat at all. Buuuuuut if he did Momo would immediately confront him, she would be extremely upset though

Kirimina

Kiri would cry and go to Bakugo. Bakugo would try to cheer him up and then goes to find Mina, thirsty for blood

Mina would cry and go to Bakugo. Bakugo would just listen to what she has to say and then he'd call Ochako or Aoyama to keep her company and comfort her while he goes to find Kirishima, thirsty for blood

Kamijiro 

Jirou would scream and yell at him and then she'd leave and never come back

Denki would cry when she wasn't there and the moment she gets back he pretends everything is fine. He tells the rest of the Bakusquad as well. And Bakugo goes to find Jirou, thirsty for blood

Iidamei

Iida would not be able to handle it. That would be devastating to him

Mei would probably bury herself in her work to the point she is malnourished and completely exhausted

But let's be honest, neither of them would actually cheat on each other.

Tokotsu

Tokoyami would just lock himself in his room and cry

Tsuyu would immediately consult him and try to talk things out. She would be in tears the whole time though. Dark Shadow would wack him as well.

Ryushot

Shinya would pretend he knows nothing but he is bad at it and she can see his anger

She would be so mad at him. Immediately just leaves

Miruhawks

If Hawks ever cheats he had better run and pray. But nothing would be able to save him at that point

Hawks just doesn't know what to do

Erikota

Eri would cry and go to Aizawa, and he would go to find Kota, thirsty for blood

Kota would just be so angry he wouldn't be able to handle it

Hitorei 

He would become so depressed 

She would get a voodoo doll for whoever he cheated with. And she would stay in her room for a long time

Toodles! X3