But majority of you guys keep quoting that, "she cheated! She cheated!" But I did not see at any point in the story that Liz and Maki confirm that they're in an 'exclusive relationship'
Romantic relationships are generally exclusive. That's the norm, specially in Japan. So unless there's a specific mention suggesting otherwise, I think we can safely conclude it was supposed to be exclusive.
Mmm nope. You can't just make blanket generalizations like that.
"Something works generally in a certain way" ==> "That's the norm everyone must adhere to" is bad, flawed logic.
And the fact that Liz herself said "I'm the worst" further confirms that what she did was a betrayal towards Maki and she knows it.
Liz seems to feel that her actions could hurt Maki, that's for sure. Doesn't mean it's a breach of contract.
But I do agree that she obviously regrets what she did.
I just hope Maki finds out so we can see some real consequence from Liz's actions.
That depends on Maki's reaction if/when she finds out. And Maki's reaction is anyone's guess. I, for one, won't make any predictions.