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 are 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 over time, leading to Yasuko hurting Fumi, Kyouko and herself in the process. Therapy, anyone? Akira is asexual and homoromantic
She doesn't seem to know very much about her sexuality, and definitely spends less time thinking about romance at the beginning of the story. Also, after Fumi brings up wanting to have sex with her, Akira says several times that she's not sure she loves Fumi the same was Fumi loves her. What if that is because she doesn't feel sexual attraction? She didn't have that feeling to go off of, so she never really thought about crushes, and her uncertainty over whether or not her feelings for Fumi are real causes them to break up. During this time, she becomes more secure in her (lack of) sexuality and realizes she truly loved Fumi romantically all along, and that is why she takes so long to confess again.