I just want to point out that Koruri and Moka have made more progress in their relationship in one chapter than Sakurako and Kasumi has made in almost 50... -_-'
To be fair Sakurako and Kasumi basically went from 0 to 100 within the first few chapters.
If you skip all the early stages and go right to happily married there's not much more progress to make^^
Being more serious the impresion I get is that Kasumi is simply Asexual and just not interested in more than cuddling.
If you look at it from that perspective their relationship makes a lot of sense Sakurako would probably be fine and willing to go further but she can tell that Kasumi isn't really interested in it.
They're relationship is clearly romantic something that is acknowledged by basically everyone arround them but it doesn't have to be sexual for it to be vallid^^