She's trying to drive her away without revealing her actual sex, because she determined that the poor girl's ego might suffer too much damage from discovering that she couldn't even tell that she was talking to a fellow female. I can see the logic; it should be less harsh of a hit if she was simply mistaken about Kanda's personality, since Ooguma hasn't interacted with Kanda before nor has seen enough from her to know what she's like.
Ookuma...You've had cream sticking on the side of your mouth for a while now.Now... how do I make her hate me...?I love him so much ♡K-Kanda-kun is so unexpectedly bold...He ate the cream that was on my face...!He ate it!!He-Come on. You're supposed to be the elder here, right?Ah!Here.Not there.Eh!? No way!!Eating a parfait is fine and all, but...