Yasuko really loves Kyouko a lot more than she lets on
Think of it: if Kyouko really gets on Yasuko's nerves so much, why does Yasuko still allow her around? You'd expect her to brush off Kyouko much more forcefully. There's also little hints in the interaction between both characters, the way Yasuko looks at Kyouko even while she says the hard things she does. It could be that Yasuko does not want to get Kyouko in trouble by starting something with her, especially since Kyouko is engaged. It could even be that Kyouko started to fall in love with Yasuko because Yasuko kept littering all these little hints around. Yasuko's crush on Kagami could be an effort to distance herself from Kyouko, which went way out of hand as time passed. Yasuko's affair with Fumi also makes much more sense in this light: once Kagami was out of the way, Yasuko needed a new foil for her feelings. Yes, Fumi is a girl, but at least Yasuko won't get her into the same amount of trouble as she would Kyouko. It's a very skewed case of "I Want My Beloved to Be Happy", which went all awkward and pretzel-shaped in time, leading to Yasuko hurting Fumi, Kyouko and herself in the process. Therapy, anyone?