So is there already a romance or [not] Kasumi? You are not making this easy.
I feel like this is kind of the crux of Kasumi's character arc right now; figuring out the exact answer to that question.
However, even if she's kind of trying to define what her relationship is with Sakurako right now, she's at least intellectually aware that if anything in her life would be defined as romantic, it's DEFINITELY her relationship with Sakurako. Hence, it's not a huge logical leap for her to think of office romance as immediately with her.