This is why Sage is my favourite, and no one will be able to dissuade me of this
Yeah, early (ep1/2/3) Sage is a weird one: without the knowledge of who she is in the future, she comes off
very badly as an angry, bitter, manipulative person. It's the additional information you get from spending significant time with her that lets you understand all of that (ie, spending 2+ years in a stifling mostly joyless relationship, especially for one as physically affectionate as Sage is).
As for the romantic vs open stuff, I don't know if Sage is necessarily being open, as in an open relationship - she's exploring since she's been in what was basically a sexless relationship for a good long while, and she is a very sexual person, so now she is trying to cram in
the MC as much college stuff as possible. It's not the same as cheating, or even being a fully open relationship: we've only had the Quinn threesome so far, though we can assume more are coming, but then (and presumably in the future) we had both MC and Sage agreeing to it and present - it wasn't one made the decision and the other went along with it, you're both asking each other if you want it and you both agree. If that's not present in future threesomes then it could become a problem.