Both girls(especially Misono) are having a tough time making good decisions about this relationship due to comphet pressure.
This is one of the few instances where cheating is actually understandable. Until Misono can gather up the courage to rebel against the heteronormity that runs her life, all she can do is either:
A) Suffer a life of abstinence while completely burying her desires
B) Cheat on a guy she doesn't love and is dating solely because that's what is expected of her