That's my point. She's a proud person with Chad and others. Her problem with Chad is that Chad treated her like a fool and dumped her. A problem of ego an pride, not that she misses him because she was in love. But if MC rejects her multiple times, or dumps her after even proposing to go to dates with her and start a relationship, she's totally OK with that? Why? Not because it fits well with Sage's personality, but because DPC needs that she behaves differently with MC for the sake of the plot. Sage's story and general behavior with MC only makes sense if MC is in her path. If not, this character is just a "plot service". DPC makes her do whatever he needs for the sake of the story, even if it doesn't fit with her personality.
And about the kiss between Maya and MC, her reaction if MC and her and involved in a relationship is clear: she's concerned. If she were just thinking in a threesome with Maya, she would be smiling to the gossip, like she does if she's not in a relationship with her. It makes all the difference. The same that Zoey being invited at the party.
On the other hand, to think that if Sage is in a relationship with MC, is going to want a 3some with her and MC is stupid. When MC goes to live with Sage, he says to her "Maya lied to me, or kept something from me". Obviously now she thinks is her lesbianism. And she knows that MC had a crush on Maya because MC recognizes that when they are in the pool and with his attitude during the "movie night". What Sage thinks before the talk with Maya is "MC had a crush with Maya, then Josy came, Maya told MC that she was her girlfriend, and broke MC's heart, but now they seems to be friends, so it's OK if I befriend Maya too.". But after talking to Maya, she knows there were something more. Do you think that Sage, if is starting a relationship with MC, is going to try to organize a 3some with her boyfriend and the girl he had a crush with only a few weeks ago? A girl that is into a serious relationship? And knowing that her soon-to-be-boyfriend and Maya had a real story, something that even made Maya to have doubts about her lesbianism? I´m sorry but it's absurd. No girl in the world pushes her boyfriend into the arms of a girl he had feelings for, and reciprocated, unless she's stupid or wants to be dumped.
The same way is absurd that Sage would organize a 3some with a girl that could bring problems to MC with his brothers: Heather, Josy, Elena and Maya. Do you think Derek would be OK seeing Maya doing "the walk"?
I don't think DPC will go that way. It would be stupid.
Sage's behavior is not a necessity for the story; probably all LIs will in some way sooner or later give MC a second chance, even those who seemingly ended things harshly with him like Bella and Jill. so there was no need to emphasize this complete submissiveness on her part.
DPC chooses to have her behave this way, perhaps to balance other more uncomfortable aspects of her character (the fact that she has long refused to admit her "romantic" relationship with MC, the teasing about sexual performance...)
even at the party, Sage was the one who had the most reason to confront MC anyway; in the end, the party is more Sage's project than MC's. from this point, the presence of Bella and Jill, if not romantically involved with MC, is much more forced. but DPC chooses that only Sage continue to flirt with MC anyway, even though she knows he has chosen someone else, who might also be there. this is all part of the characterization DPC wants to give her, always taking into account his limitations.
on the threesome talk, it's probably just a DPC joke, but it's undeniable that he wants players to think about it, first the kiss, then Maya saying that she really likes Sage, and finally Sage opening up to the possibility of "many more" threesome.
theoretically Sage thought that Quinn also had feelings for MC, and Mc may have even confirmed it to her, but that was not a hindrance at all. the lying thing is all in the past, now erased (in this chapter Maya and Josy even explicitly admit that they lied), at BR they are all goldfish, the past is always far away.
by now Sage cannot be afraid of Maya and Josy from that point of view, they are girls like the others (in fact she has no problem with them, at the limit it is Zoey, who is a newcomer who raises some doubts for her), definitely she doesn't trust them as she mistakenly does with Quinn, but other than that they are now HOTs like the others.
the walk of shame then cannot be a problem and if it was just do it again not in the MC room but at the HOTs (as happened with Quinn and for a HOTs it remains the easiest solution)
I repeat: it is probably a joke by DPC, but the idea he wants to leave is this possibility.
if we really want, the fact that Sage knows about Maya and Josy's relationship makes the kiss even weirder, because it's one thing to kiss a friend as a joke, it's another to kiss a friend who you know might be particularly sensitive to that kiss