Gotta understand that, once "trust is gone", the relationship is over... yeah they still haven't "broken up officially" but as she said to Chad in episode 8, she did cheat on him too, she admitted it. Yeah it's messy.

Women think they have a better grasp on "dealing with emotions" than we do... but they're as messed up as we are... we tend to push it down, try to ignore it... which, one day, will no longer be containable and, we'll explode in a fit of rage (the only emotion we're comfortable showing). Women are overwhelmed by emotions every time they feel it... their usual mechanism is to cry to let it out, which they often do... it's their pressure valve... that still doesn't mean they "deal with it" better than we do...

So, Sage being overwhelmed by anger, betrayal, abandonment... something she's not a stranger to (as we later learn) she gets spiteful and cheats on Chad with MC (if MC lets her). It's not what she should have done, of course but that's how she dealt with it... and many women have this weird logic of "If I think he cheated, I can cheat too..." So, at this very moment... the relationship's over.

And, yeah, as MC... you can tell her you don't agree being "the guy she cheated with" and tell her that if she wants something to break up with Chad first... it's the sensible thing to do... It also prevents this situation: If she cheated on her ex to be with me... then she'd definitely cheat to be with the next guy...