If you read that first sex scene and didn't feel queasy about Ruki, then I absolutely can't comprehend how you're approaching this comic.
That one frame was certainly a danger sign, but it also had no context. We don't know how they ended up that way. All the scenes between them leading up to that show Ruki being very non-pressuring about, well, pretty much everything. You could assume that at some point the mangaka didn't show us, Ruki completely changed her approach. Or you could assume that Sachi was so not emotionally invested in sex or not-sex, and so not emotionally invested in Ruki, that she decided (as she decided in a few other places) to just go with the flow and take an easy way out instead of communicating. It's very ambiguous and IMO not that well handled by the mangaka.
You're also contradicting yourself, since you acknowledge that they never had an explicit, agreed-upon relationship, and yet apparently believe Sachi violated Ruki's trust by sleeping with Remi, which doesn't really work.
Oh, come on. Ruki made her side of things clear: She loved Sachi, wanted to go out with her, wanted to be in a loving intimate relationship. Sachi proceeded to go out with her and have sex with her. She never said "By the way, I'm just playing with you to pass time and distract myself from the bad shit you helped me out of"; both she and everyone else knew perfectly well what Ruki would draw from that. Even she didn't pretend it wasn't cheating, was quite clear that that's exactly what she considered it, so I don't see why anyone else is saying different.
I keep saying this, but it's absolutely perplexing to me: What is WITH some of you people and cheating? This rigid heuristic CHEATERS ARE BAD and so an enormous focus on that as the root of problems when that's not what's going on.
No, cheating is not inherently bad. I've known people who've been cheated on, and it wasn't that big of a problem in their relationship, just because they didn't care too much. I've known people who cheated as a way of ending a relationship where they weren't being treated well and they were too scared to end another way, and while that's not exactly noble, it was absolutely a good thing that they did it and the terrible relationship didn't continue. Things are complicated.
I'm finding it hard to fathom your perspective myself. Let's take it this way around:
Betrayal of trust is bad. Can we agree that far?
Cheating is a betrayal of trust. That is, it involves breaking a compact and going behind the other person's back to do so; generally although not always, it involves lying to the other person or at least concealment from them of information they would consider important and very relevant to your relationship. Now a "going out" relationship does not have to be monogamous, but in our society unless you get explicit about it being otherwise, it's fairly definite that it will be and people are going to believe it is. So there's a compact there which the other person is trusting in. If you break it, and all the more if you deceive them (which Sachi did for a month or so and if Remi hadn't decided she wanted to see Ruki's heartbroken face we don't know how long she would have kept the deception going), you have betrayed their trust. If you have an open relationship sleeping with someone else can be not bad, but then it's not cheating.
It is worse the closer the person you betray is to you, the more they care for you and so forth. Of course if you're "with" someone who doesn't care about you, with whom you are not close, to whom you owe nothing . . . then it's a more minor betrayal of trust, there being little trust to betray, but you shouldn't be with them in the first place. The default case is there's some reason you're together, no?
What you say to defend cheating is really about ends justifying means. It's my opinion that if you do something bad for some reason that involves a greater good, it may be necessary but it is still bad. The CIA may murder someone because they think lives will be saved if they do (OK, that's not really why the CIA murders people, but you know, if it was), but the act would remain murder and it would still be bad. That crime would be on the conscience of whoever did it whatever the ends they served. So OK, there may be times when due to someone's weak personality and lack of allies, coming up with a move like cheating is their only effective way out of a terrible relationship. But it would still be better if they could have come up with a more honest way. The cheating itself remains bad, even if there's some odd corner case where it's a means to good or necessary ends.
Hurting people is also bad. Usually, cheating does. There may be times when cheating doesn't hurt the other person. It's still a betrayal in such a case. But anyhow, that so does not apply here. Again, I can't see why anyone is trying to argue that Ruki wasn't hurt. Exhibit, for example, aside from all the tears and such, page 21. Sachi says "I'm going to tell you something that'll hurt you" and Ruki's answer is, "I've been hurt too much already, there's nothing left to hurt."
I'm already destroyed, it's too late to hurt me more . . . I don't get an analysis which wants to claim she wasn't hurt.
