I don't even think Sage is a slut, actually. I think it's a facade she puts on because she thinks that, as a "hot chick", that's the way she's supposed to be. But what little we know of her actual history, she's had a steady boyfriend for years who she seems to legitimately dote on (or did, at one point), and her one known attempt at having a simple "fuck buddy" has already shown signs of catching feelings despite it being mere days since they agreed on it. Basically, I think Sage is more of a tease than an actual slut; sexually liberal, sure, but only if she lets you in, and even then she doesn't seem to separate sex from emotions all that well.
Of course, I could be completely wrong about all of this, depending on what else we find out about her and her history, but that's my read on it 5 episodes in.