I think you can say their relation is a platonic love. I think both of them, Kasumi mostly and Sakurako to a lesser extent, are not that much interested in a more physical relation . Remember we talk about 2 girls who don't see the point of putting their love into words because they know how important they are for one another so i can assume they're not that much intested by that.
There's a difference between not bothering with words and not wanting a physical relationship, although I do admit there's been no evidence so far of them having one. Even just kisses are still something quite rare. I really don't think it'll stay like this forever, but they're simply growing their relationship at their own leisurely pace.