How would she be testing us? Sage has shown ZERO interest in MC (romantically speaking). Wanting to be friends with benefits with her is pointless when you could have just accepted Quinns offer.
To be friends with benefits, you first have to be friends. Friendship implies interest, with or without
benefits.
Sage likes the MC because he has helped her find out about Chad, and because he has shown an actual interest in her by teaching her to play the guitar and by playing it for her when she is down. He has been there for her when she needed someone.
I see Sage as quite sympathetic. Her boyfriend has been proven unfaithful and she is genuinely upset by it. She has rushed into rebound sex with the MC, if you played it that way, but is not ready for another boyfriend so soon after Chad.
Everyone experiences the NPCs in this game differently, so I am not trying to change your mind here. Just expressing my own opinion on the matter.