It wouldn't take an explicit sex scene to indicate that K&S sometimes have sex, if maybe not as much as S would like. Or that S had resigned herself to worshipping and serving the asexual Kasumi. Some words would do; they could even be thought bubble words, the manga uses those. But the mangaka has chosen not to insert such clarity, and so we're left wondering, and worrying if there's some unresolved tension to the relationship -- yes, they're definitely in some sort of relationship, and S has used words like "my girlfriend and pet" without getting smacked, but no, it's not clear that she won't snap from frustrated lust some day.
In the original oneshot you Will find one of the most explecit parts of this series... Sakurako comments on Kasumis bust size
Given that the author decided to Not repeat that scene in the serial version its probably a fair assumption that we Arent gonna get anything... Well in the Other hand we see Sakurako squashing herself between kasumis legs 100 times and the first time she gets drunk she imediatly tries to make out with her, i might have contradicted myself