Lee isn't the only one hurt and disappointed by her cheating. In a self-depricating moment, Mary Jane called herself a "selfish b***h“ who "tanked a perfectly good relationship for no reason." (That last part is pretty accurate.)
Meanwhile, Justin didn't lose any sleep over the cheating drama. In fact, he may be more interested in the thrill of the chase, than actually falling for Mary Jane.
Despite questioning whether he's the "devil," the man of her "dreams," or a combination of both, she decided to tell Justin that she split with Lee.
To put it mildly, Justin was less than excited to hear the news. Especially because (surprise!) he has a girlfriend, and he's not willing to risk the relationship for a “whim" (a.k.a. Mary Jane).