Why does desdemona object to the way othello has agreed to "deny (her) nothing" and see cassio? What does she want his acquiescence to mean but it does not seem to?

Desdemona decides that she wants to advocate for Cassio. She tells Emilia so, and that she believes Cassio is a good person, and has been wronged in this case; she pledges to do everything she can to persuade her husband to reinstate Cassio. Cassio speaks with her briefly, but leaves just as Othello enters because he does not wish for a confrontation. Iago seizes on this opportunity to play on Othello's insecurities, and make Cassio's exit seem guilty and incriminating. Othello then speaks to Desdemona, and Desdemona expresses her concern for Cassio; she is persistent in his suit, which Othello is not too pleased about. Desdemona pleads for Cassio which irritates Othello even more. Desdemona resorts to reminding Othello of his promise to "deny her nothing". Desdemona expects Othello to acquiescence to hear Cassio but Othello merely puts her off saying he will deal with it another time.