Why does lago not tell Cassio that Othello is married to Desdemona? Act1

In Act I, Scene II? Iago is playing stupid.... he wants Cassio's job and feels it was taken from him. Iago wants to set Cassio up in the same way he is setting Othello up. It is Cassio who first mentions the marriage, although, Othello enters the room before he can say who Othello has married.