If Emma had politely refused him without giving a reason, Mr Knightley would probably think he had been too sudden, that she had thought of him as a family friend, almost a relative, and needed time to think of him as a lover. Perhaps like Edmund Bertram at the end of MP, he would think it "an hopeful, a possible undertaking to persuade her that her warm and sisterly regard for him would be foundation enough for wedded love." (Edmund did not know that Fanny's heart was already his!)