A woman asks the speaker to pretend that something between them never happened. It sounds like a request for a clean slate, but the speaker turns it around in the second line: there's no need, because she never treated it as real in the first place.
The reply is short and cutting. The speaker doesn't shout or plead; he states it flatly, almost like a verdict. That calm is what makes it sting. Underneath it, you can sense hurt. Whatever happened mattered to the speaker, yet the other person behaved as if it meant nothing, and now she wants to make that official. The speaker's answer says, in effect, you've already been pretending all along.
The mood is cold, bitter and a little weary. There's no warmth left, just a quiet, sharp acknowledgment of being let down. The brevity of the poem mirrors this: nothing more needs to be said.