I don't think Sakurako's comment about them "not dating" is a statement that they are not at all sexual but if it is, that's fine too. They could be in a queerplatonic relationship and that would be just fine. They do occasionally kiss and Sakurako occasionally obviously is attracted to Kasumi, but it's possible that Kasumi is ace, and Sakurako has figured this out and respects her boundaries.
We did get "partner who's something like my girlfriend, my pet cat..." https://dynasty-scans.com/chapters/a_room_for_two_ch53#5 Followed by grumpiness about labels.
That's also the chapter that had Kasumi spontaneously kissing Sakurako on the lips when she got home.
As someone else already pointed out, it was probably a cheek kiss, but it might have been a poorly drawn kiss on the lips as well. We can dream. That chapter ALSO has Sakurako taking Kasumi a drink at work at the photography studio and seeing the picture that her boss took of her in a wedding dress hanging on the wall and gushes over it, saying, "I'd love for us to wear matching wedding dresses!" Kasumi asks, "For what?" and Sakurako says, "A wedding ceremony!" Then, in chapter 59, there is this little exchange:
https://dynasty-scans.com/chapters/a_room_for_two_ch59#9
Sakurako wanted to get thumb rings. After they buy them, Kasumi thinks to herself, "But anyway, I thought she was gonna say that she wanted rings for our ring fingers. That's surprising..." And Sakurako thinks to herself, "Even if three months pay is expensive, I guess I'll do that after I start working full time." Then Sakurako SAYS: "I'd love to go back there for some thinner rings!" and Kasumi responds, "Yeah, one day."
So it's clear that Kasumi totally expects that they will have a commitment/wedding ceremony of some sort at some point and wear rings on their ring fingers. SHE is not going to initiate the process, but she will be happy to wear the ring when Sakurako decides it's time. Regardless of anything else, they are life partners.