Explain what happens between Iago and Cassio in the short time Othello is off stage in the Sagittary (Lines 47-52?

Iago pretended to be supportive of Othello's marriage to Desdemona, but when Cassio enters, he uses a rather uncomplimentary metaphor to describe the coupling. "He tonight hath boarded a land carack," Iago tells Cassio; his diction and choice of metaphor make Othello into some kind of pirate, stealing Desdemona's love, while reducing Desdemona into a mere prize (I.ii.50). But, this tone is carefully calculated; Iago will soon want Cassio to think of Desdemona as an object to be taken, and to believe Othello to be less honorable than he is.