Sage suffers badly from her attractiveness being taken for granted. She was initially introduced as the queen of the party girls: president of the HOTs, girlfriend of the biggest man on campus, sexy redhead 'bad' girl, the works. Naturally, such a girl would be extremely desirable. As time went on and we learned Sage's actual story, it turns out most of those initial impressions were incorrect (or at least very misleading): Sage is committed to a completely sexless relationship, she struggles to form emotional bonds with anyone, and most of the HOT sisters are way less inhibited than she is.
But despite this dramatic change in how Sage is presented, it seems DPC never really thought about what that would mean for Sage herself. Sage was originally intended to be confident, so she remains confident despite no longer having anything in her backstory that would justify such confidence (and several things that would contradict it). She'll give the MC an important lecture in how life outside a bubble works despite, it turns out, actually having far LESS understanding of how her sorority works than Rusty did of the DIKs.
I don't think we're supposed to notice these contradictions. I think we're supposed to accept all our initial impressions of Sage as valid, and then consider the specific circumstances as unique exceptions that have no larger implications. It's a bit like the Affinity system: we're supposed to accept the overall 'feel' of the setup even when the details wind up contradicting it.
It's one of DPC's biggest weaknesses as a writer, IMHO.