Dear Mangaka;
When you have a relationship like this (Sayuri is... 16-17?) please don't treat sexual attraction in a reductive manner. And please stop treating mere social dates like freaking marriage. The author could have gotten mileage out of Koyuki saying, "well, look, I'll agree to date you, but not exclusively or anything. You're young enough that you should be talking and seeing more people." And then they could date, and kiss, but leave room open for Sayuri realizing it's just a happy girl crush.
Sexual identity is destroying the human race, I swear.