I don't know. I agree Sage doesn't seem especially interested in girls, and I don't see her getting into any sort of long-term poly situation. But I could see Sage being up for a light foursome where the three girls fool around with the MC a bit. Probably not to the point he has sex with all three, but somewhere close. Sage does claim to have a kinky side and I think she'd enjoy teasing the MC a bit.
Even then, I don't see her "kinky" side applying to stuff involving other girls or them doing things with the guy she's with/crushing on. We've got Maya, Josy, Quinn, Riona, Nicole, Sandy, and possibly Camila, Lily, Sarah, Melanie, Jade, and Cathy to satisfy the girl/girl itches and don't really need all of them to have a sapphic side. (And yes, I say this still fully believing in the Bella/Jill theory, but that just makes me feel more that it would be best that Sage doesn't have a sapphic side.)
I think that at this point of the story (Ending of Ep6) Sage, in my opinion, is in between both of your opinions stictly because shes now starting to contemplate the MC as "Possible LI" and not just a "Fuck Buddy" like she did in the beginning of their arrangement.
She does have a kinky and exploring side that we got to see through the episodes like the Control or Domination choice, the asshole tease that lead us to believe that she would be down to do some Anal when the place is more appropriate (
The DIK library hopefully
) and the Forbidden/Sneaking around. Thats the side that would accept a threesome or foursome but only with girls she likes and trusts to explore that lesbian side and to experience group sex
(For Example Quinn, because she already knows and caught them having fun and because Sage throws the MC over to her in Ep4).
However, the Sage that is now Single and contemplating the MC as a "Possible LI" probably doesnt want to "share her food" and if she did that would be a good way DPC could build Sages jealousy and maybe makes her want to take the relationship to the next step
.
But seeing as the game is FAR from finishing, I think we will have a lot more of the Fuck Buddy Jealous Building Sage
, than the Relationship Dating in Love Sage
In other words, the transition between "Its just Sex and People put the pussy in some
unnecessary pedestal" to "Sex must be with someone youre Dating or Love" is a LONG WAY from happening