I don't think that a proud girl like Sage, that knows that could attract any guy in B&R could be OK with a freshman rejecting her several times and/or dumping her after proposing to her to start dating, and she accepting. I don't think that Sage would hate MC, but I don't think that she would insist in doing things with him like planning the party, hanging out together, or being so friendly and flirty with him in the party. I think it makes little sense. It's just that DPC needs that MC and Sage remain friends because he needs it for the sake of the plot, and she's friendly and flirty with him because DPC uses a good number of the same recycled scenes for Sage's boyfriend path and for Sage's acquaintance path, only changing some details.
Actually, I don't think that MC would be friend of a girl that for at least two episodes is a bitch with him. She blackmails him threatening not to return a guitar that is obviously his, and doesn't care if MC is going to have a lot of problems with Chad and the tri-alphas because of her. The logical reaction to MC once he gets his guitar back is to think "That Sage is just a selfish bitch" and forget about his promise to her. It was made under blackmail, anyway.
I think that the first two episodes of Sage are terrible writing, and all the story of Sage as just a friend of MC has mostly cero sense. I don't see why they could become friends without having the "fuck buddies" relationship. But it happens the same with other characters. Bella, for example. I don't see how a guy like MC that spend the two or three firsts episodes being an immature asshole in front of Bella could become her boyfriend at some point. It's the problem of a story so complex and with all those branches. Some of them work, some of then, don't.
