I wasn't making an argument that Sage is justified in cheating. If your partner cheats, and you cheat in revenge, that's still cheating. My argument is that from Chad's POV there was no relationship for Sage to cheat on.
Thought experiment - let's say Chad finds out that while he and Sage were going out, MC was giving her the foot long special. Does Chad think Sage is a cheater? No, of course he doesn't think Sage is a cheater - in his mind, he wasn't in that relationship anymore, he hasn't been together with Sage since he discovered he was gay, it was all a lie that he dragged her along for. The only reason he pretended to care was to keep up appearances. Does Sage think she herself is a cheater? Obviously yes, because from her POV she was still in that relationship, but she wasn't. Again, the relationship didn't exist for Chad, it was a farce. You can't cheat on someone who isn't going out with you.
I very much disagree: Chad may not have been emotionally invested in his relationship with Sage, but he was well aware it existed. As was Sage.
If Chad found out about Sage's infidelity, I don't think he'd be angry the way Sage was by his, but I do think he'd care. If nothing else it reveals Sage is a bit of a hypocrite, so he could lecture her about punching him. But it's also possible he has an oversized ego and would still be offended at the idea anyone would cheat on him even if he was just going through the motions. People aren't always rational. Plus, as I said, Sage
tried to cheat on Chad before she knew for certain he was cheating, it's just the MC and/or Jacob managed to stymie her attempt.
In short, while I think Chad is unlikely to have been emotionally hurt by Sage cheating on him the way she was (or the way the MC theoretically was by the whole Maya/Josy reveal), that doesn't mean the relationship didn't exist. It did,
especially for Sage, and she still chose to sleep with the MC rather than confront Chad and either work something out or end the relationship honestly. She cheated, end of story.
I don't see her situation as much different than Josy, at least in the setup. Neither girl was getting what they needed from their relationship, and both saw the opportunity to get that from the MC instead. So they slowly rationalized a way to have a fling with the MC anyway, only for drama to ensue because that's how fiction works.
The main differences are that we know there was genuine affection between Maya and Josy at one point, and that Josy seems better able to accept what she was doing. Josy didn't pretend the MC was just a roll in the hay, and if it winds up being a one-time thing she shows a desire not to repeat her actions in the future.
With Sage, I think she's still unwilling to acknowledge what she did with the MC and what that says about her and her feelings for him. Some of that may come down to DPC trying to limit the branching for her paths, but a lot of it is just Sage being unable to cope with Chad not wanting her.
Which is not to say Sage is a bad person. She was a young woman in a bad situation and she made a mistake. I think she'll work her way through this eventually. But she did cheat and she will need to acknowledge that at some point before she can put this all behind her.
Do we have any evidence that Sage and Chad ever fucked? I can't remember the specifics, but I do recall Sage saying that there was at least quite a prolonged time period of no sex. Maybe they never even did it. Speculation: Do we really believe that Sage didn't fuck anyone else before the MC, especially given how horny she is? Also, factoring in very little, even perhaps no, sex from Chad? I'd speculate that it's unlikely that MC was her first instance of disloyalty. I can't prove it, of course. It's just a gut feeling. That girl likes sex and I don't think she'd hold out forever.
We have no indication there was ever affection between Chad and Sage at this time and I'm inclined to think the relationship was always pretty passionless (though it's true that absence of evidence is not evidence of absence). We also have no proof whether Sage has cheated on Chad prior to the MC. In this case, however, I'm inclined to think the MC was her first since she started dating Chad.
First, her inner monologue where she decides to play around with the MC in Episode 2 suggests she needs to rationalize this because she now knows Chad is cheating. That makes a lot more sense if this isn't something she's done before. Also, when Jacob drags the MC off Sage at the party, he mentions that doing anything with Sage in public will cause big waves very quickly. Granted, that could just mean that Sage's previous flings were smart enough to be discrete. But given how little effort Sage is putting into hiding her affair with the MC, I'm skeptical she would have been more clandestine in the past.
I think the easiest explanation is that Sage had an active sex life before she started dating Chad, but had been loyal to him since then before she met the MC.
It was fun for a non fuck buddy run, but you're right that option doesn't really make sense. If I had to think about a reason then it's probably because DPC noticed how popular she was so now everyone had a chance in ch7, and it worked her popularity is through the roof. I just hope this was a one time thing and there will be no more shortcuts like this in the next chapters.
I'm pretty sure DPC always intended to give Sage an opening for people who didn't want to cheat with her, just as I'm sure Maya and Josy will eventually have solo routes. The question is why the two paths converged so suddenly.
I'm hoping it was just DPC trying to conserve art/coding time by reusing assets in both paths in the already crowded Episode 7. But if so, I think this was a situation where the savings aren't worth it. So hopefully going forward the two paths will branch back out a bit. My fear is that they'll basically stay interchangeable, and the the new branches will be about just how interested the MC is in a long-term relationship vs a short term FWB one.
Those are fine branches to explore, but fuck buddy/friend should explore them from two different starting points.