A question about High School of the Dead?

When Saeko is outside the door of the room where Rei and Takashi are talking about their envisioned future relationship, what does Saeko mean by: "You should really not call me 'Miss' because you know we're...." and what does the reaction from Rei show for it: "I know...I don't want to hear you say it, cause I don't wanna hear it."