Cecily and Miss Prism's main topic of conversation always is Ernest, Mr Worthing's wicked brother. So for the girl the men who are talked about many times are attractive, and she is in love with Ernest.
Algernon (Ernest in that moment for Cecily) tells to her about his feelings, as a consequence o that, he decides to propose to her. But, in that moment, Cecily says to him that he proposed to her three months ago. That means that, since that time they have been engaged. Algernon doesn't know about that because he didn't propose to her. Three month ago Cecily proposed to herself and she pretended to be Ernest. That surprises to Algernon when he proposes to her. Really is this the answer?