I'm pretty sure Sachiko's an outright lesbian. The whole aversion to men thing is just too convenient and sounds like a cover story to me.
Given that, and that Sei repeatedly states she is a lesbian (and was suprised when Suguru wasn't apparently 100% gay like she was) and had a girl as a lover - then no wonder Sachiko was so easy to freak out over Yumi.
Contrariwise, I wonder if Yumi's transfer of so much affection to Touko was Yumi's way of "growing out of her girl's love phase" since, frankly, it's a big change in her character. Yes, she loves everyone, even her enemies, but her "love-love" had always been, first and foremost, Sachiko
Maybe Konno-sensei decided two-and-a-half lesbians (Sei and Shiori and a closeted Sachiko, or 3 ½ if you count Kanako) was enough. I always hoped Sachiko would marry Suguru and Yumi could live with them as Sachiko's secretary and "close companion."