I know what you meant - for the most part I am in agreement, but you seem to miss what I meant.
Sage is not actually focusing on 'the one' single guy at a time, because she could have decided to cut off Chad the moment she felt attraction towards MC or anyone else, let alone at any time thereafter. Instead she clings on to her evidently mostly nominal relationship with Chad, finds it in her to go ballistic and vindictively investigate who his side bitch is - why isn't she walking? - while fucking her tingles away with MC coz its just sex and its not the same, slowly catching feelings in the process. That's her straddling two guys, in more ways than one. And it would fit that she does that precisely because it's out of her character - feeling attraction towards two men at the same time is the unexpected event in her story, a catalyst prompting her to make an unusual decision that leads her into an unforeseen and thrilling adventure.