Thing is, we know absolutely nothing at all about the off-screen relationships these women have with their respective girlfriends.
Maybe Azusa's girlfriend cheats on her, too. Maybe Midori's relationship is essentially over already and she just hasn't pulled the trigger yet (she was pretty quick to say "Let's dump our girlfriends and start dating each other" after all). Maybe Azusa really does love her girlfriend and they have a fantastic relationship but she can't resist the thrill of infidelity. Maybe her girlfriend knows and puts up with it.
There are so many possibilities of what could be taking place off-screen in these relationships, and the text of the comic itself gives absolutely no hints about it whatsoever. So casting the whole thing as "horrible people engaged in betrayal" is pretty reductive.
And that's not even getting into the whole "people who cheat are automatically horrible people" bit. In my experience, that's not necessarily true. People are complicated and messy, and normal everyday people cheat all the time. Sometimes they feel regret and remorse, sometimes they don't. Doesn't make them horrible. Just flawed.
For me, I find the story intriguing because of the central conceit: That there is such a powerful chemistry between these two women that they can't help themselves. That's an interesting theme to explore, and this story does it well.