Implying it would've been less terrible if she'd cheated with another woman?
To this day it still confuses me how the default assumption when a woman in a gay relationship starts dating a man is that she was actually straight all along and not that she's simply bisexual. You know that's a thing, right?
People tend to see cheaters as deceitful in every way possible. "You never really loved me" is a very common thought for someone who has been cheated on. If the cheater is not openly bisexual, people will think they have been deceitful even about their sexuality, just like when a man cheats on his wife with a guy and people think he was gay all along.
The evil/cheating bisexual is such an overused trope already, I don't know why we just expect/assume even more of that.
"Implying it would've been less terrible if she'd cheated with another woman?"
It's certainly not less wrong, but it can feel less terrible (even if just a small amount) to some people. If your same-sex partner cheats on you with a person of the opposite sex, there's a higher chance of you not only thinking things are wrong with the cheater but also thinking that things are wrong with you for not being of the "right sex", thus leading to strong feelings of inadequacy.