Sakurako just didn't have enough confidence in herself.
At that point in the story they have been sleeping together for a long time but they still are kids in school -- and you know how graduation is the time when you turn into an adult and are expected to change your life: stop playing around, accept grown-up responsibilities, ditch your school lesbian sweetheart, find a man, marry him, have his babies, etc. etc.
It's not at all unreasonable for Sakurako to think that an amazing beauty like Kasumi isn't exactly serious about her, and would rather be just friends from then on and go find herself a millionaire husband in the future.
Sakurako's lines at the end can be read as: "Hey, you really mean what you said? You don't want to be the trophy wife of a wealthy sports star or something? You chose me instead? Like, for realsies? I'm not dreaming?"
Heck, you've seen how, years later, she still has trouble believing the Kasumi at her side is real... 
last edited at Oct 19, 2021 2:35PM