I thought it was pretty realistic. I can understand what Ruki is feeling, and Sachi comes out as a different kind of person than expected. At least she knows she's cruel.
This reminded me of a part toward the end of Maison Ikkoku. Towards the end, Godai gets called by Akemi to a love hotel because she got abandoned and couldn't pay the check to get out. Of course, Kyoko sees them leaving and thinks that Godai was messing around with Akemi, and she gets in a fight with him about it. She then accuses Akemi, who says something like, "So what if we had sex? We're not children. Does that change who Godai is? Does that really change what you think about him? Just grow up!" Now, the situation isn't exactly the same (since Kyoko and Godai were never officially together until later in the story), but it does come to mind.
If you're really in love with someone, you're not going to stop loving that person so easily. Ruki isn't going to stop loving Sachi because of the cheating. I can see why she forgave her.
The whole point of this arc was they weren't playing by the same rules because they didn't view the relationship the same way. Sachi saw it unseriously, while Ruki was playing for keeps the whole time, desperately trying to make it work. Sachi didn't understand because she was trying to view it from the "normal" perspective, in which it DOESN'T make sense. In a girl-girl relationship, who's the "guy"? Who takes the lead? Who's the "strong" one? You can say "relationships don't have to be that way", but this is obviously a key issue or there wouldn't be the whole butch/femme thing. Ideally a relationship should not need these type of fixed roles, and I think in most longer relationships this dichotomy breaks down to a greater or lesser extent (can't say for sure because my relationship experience is basically nil), but it IS an issue.
Now that Sachi realizes she can BE the strong one, that she likes Ruki even though she's weak and awkward - even partly because she's weak and awkward - they've restarted their relationship seriously... Does Sachi deserve it? I'm not even going to go into that territory. I have a friend that cheated on her fiance with 2 guys (at the same time, as in threesome). It came out before they were married but in the end they stayed together and have been happily married for many years (and I'm pretty sure she hasn't done anything like that since). Who's to judge?