I wonder why no one is commenting on the fact that Kasumi explicitly questioned her actions and wants by saying "[...] But that's not what friends do, so." That line speaks volumes about why this hasn't gone anywhere. Kasumi is VERY detached from the concept of romantic love, to a weird degree even, which has been pointed out by the author many a times, the ONLY reason this has gone anywhere, is because The Sakurako Thirst is stronk. Also, Kasumi is obviously in love with Sakurako, but she just can't come to terms with it, because of that previously mentioned detachment to romantic feelings. It's just alien to her, so it feels weird, not wrong by any means, just... Weird.
Also, for those wondering, it has been very heavily implied that the farthest they've made it in the past... Years or relationship, is to a kiss on the lips. Yup. No more.