That's one of the interesting things about Kasumi. She is incredibly tolerant of of Sakurako's affection until she crosses the line at which point Kasumi will rebuff her and sometimes really harshly depending on how badly she overstepps it.
There are basically two lines Kasumi draws in the sand. One is how creepily obsessive Sakurako is allowed to act towards her and the other is how sexually charged Sakurako's affection is allowed to be. Kasumi loves Sakurako and doesn't mind or often even likes physical affection with her at around the cuddling level, but the moment Sakurako wants or tries to go further than that Kasumi always turns her down and corrects her, sometimes pretty bluntly.
Kasumi is always the one who actively defines the boundaries of their relationship where as Sakurako would have strait up figured out a way have her babies by now if Kasumi had allowed it.
She is certainly in control, though I would not necessarily say that she stops Sakurako from physical affection transcending cuddles or anything serious. It really does feel more like Sakurako herself is holding back and intentionally not crossing that line. She often talks about it, but in the end she never forces anything on Kasumi. Basically, I have a feeling Sakurako is waiting for Kasumi to open up and show that she wants it. There were several instances where Sakurako almost hopefully thought Kasumi would kiss her or get touchy-feely, but when she actually fawns over Saku that makes her weak and meek.
https://dynasty-scans.com/chapters/a_room_for_two_ch29#10
What Kasumi harshly turns down is this "worship" of herself. She seems to find it profoundly creepy and unecessary.
This is just my own interpretation, but I believe that Kasumi wants an equal give and take relationship. Sakurako always puts Kasumi on a pedastal and that gets in the way of them being on eye-level. Kasumi shows the most affection and attachment when Sakurako isn't being all crazy over her.