I'm sorry but this is nonsense. Let's not forget that Fuckface is going to visit Zoey
after his girlfriend fucks off to gods know where. Going with your own logic, whatever Sage was going to do was "more important than her boyfriend".
Of course, trying to infer this sort of "the thing she chose to do is more important to her than me" is complete idiocy, except it is exactly what Sage is doing. She is getting pissed off that MC is going to do his own thing, in his spare time, after she decided to do her own thing rather than spend her time with him.
Yeah, but neglecting your partner and how he might feel about it because you have more important things to do, well that is clearly fine and not even worth consideration.
(to make it even funnier, Sage does it again later in the episode, where she decides she'd rather hang around with her new friends than her boyfriend who maybe could use some support at the party he organized. As do all other main girls for this matter)