Mitsuko's irrational mistrust of Kyouko and general misogyny backfire on her. She succumbed to her insecurity and now she has what the other women wanted, but what she wants is forever lost to her.
Or maybe I'm reading too much into it. Maybe it's just a story about bad things happening to bad people.
I actually disagree, both with the "irrational mistrust" and the "what she wants is forever lost to her".
I think it was pretty obvious that Kyouko was cheating on her. Yamada was badly shaken by those rumours, kind of hard to see it as "oh no, the girl who just flirted occasionally with me at work has slept with another guy".
Kyouko gives off the "would sleep with anything that moves" vibe, something about that "I just think, 'let's do this!'" and her general flirtatiousness. Though, I am not saying flirting with people equals being promiscuous or unfaithful, just that this particular case gave me that feel, but since the one-shot is not from her perspective, this is pretty open to interpretation.
As for the "what she wants" part (as in, what Seo wants)...
I interpreted their relationship as casual sex. That is the feel I got out of it. A casual relationship that Seo fucked up, and Kyouko later realised it meant something to her. One thing that goes along with this interpretation is the implication at the end that Seo stopped seeing Kyouko after getting together with Yamada, and apparently Kyouko never attempted to talk to her. I mean, if your girlfriend suddenly stopped seeing you, and you even work at the same company, would you not try to talk to her, see what the problem is? On top of that, Seo did not even try to break up with Kyouko, she just slept with a guy and then broke contact with the other woman. Neither of them behaved as though they had relationship problems, neither of them behaved as though they even had a relationship.
I think the reason Seo got attached to Kyouko might be because she was literally the first person to accept her, to not ridicule her looks, to show her kindness instead of scathing remarks. I find it interesting that what finally drove Seo over the edge was not the "cheating" aspect, but her insecurity over whether Kyouko was faking pleasure with her. Notice how we never see Kyouko pleasuring Seo, it is always the other way around. Just look at Seo's facial expressions, how she obviously relishes in bringing pleasure to the other girl, how confident and proud of herself she looks. And then, that is exactly the aspect of their "relationship" she started doubting, in the end she was convinced Kyouko was "faking it", and that was what drove her into despair. Note that she sleeps with Kyouko after hearing the rumours, and the only thing that is on her mind during the sex is "is she faking it", not "my girlfriend is cheating on me".
Irony is that I think Kyouko is the only one who got hurt in the end. I genuinely think Seo was not actually in love with the other woman, she just got high on the power of bringing pleasure to another person, after her previous partners plainly told her she brought none to them. The moment she started thinking Kyouko was faking it, she proceeded to abandon ship as fast as she could. Kyouko's obvious sadness at the end, on the other hand, is a pretty good indicator she realised this meant something to her.
On a side note, Seo in general seems to have... gotten a really bad message out of all this. Here she seems to conclude that a girl will always respond with enthusiasm, even when faking it, so that is what she probably thinks under "the role of a woman", backed up by this scene, where I can not help but think she kept "moaning cutely" because he was asking her does it feel good. Given the previous scene with Seo asking Kyouko did she like it, Kyouko enthusiastically giving an affirmative answer, and Seo then dismissing it as faking, I think this scene mirrors that one, with Seo basically resigning herself to "fuck it, it is all fake" kind of thinking.
Or maybe I am reading too much into it.