In Othello, why does Cassio display ignorance of Othello's marriage when conversing with Iago (1.2.49-53)?

There is a very simple reason why Cassio appears to be
ignorant about the marriage of Othello to Desdemona in these lines. The truth is that he
didn't know about it. Let us remember that we are told in Act I scene 1 that Othello and
Desdemona have married very recently and in secret. This is news to Roderigo, who
clearly wanted to marry Desdemona for himself, and thus we can infer that it would be
news for Cassio too. The sudden and secret nature of this marriage would have meant that
even those closest to Othello, such as Cassio, would have been kept in the dark until
Othello chose to reveal it, or until others did the revealing for him. Therefore, Cassio
is not feigning his ignorance in the section of the play that you have identified. He is
genuinely surprised at an action that shocked many.