covered w/ a Barbary horse; you'll have your nephews neigh to you; you'll have coursers for cousins and gennets [small horses] for germans" (I.i.115-16). Desdemona "and the Moor are now making the beast with two backs" (I.i.117-18). The message is charged with racial epithets. To have been carried off is one thing; to have been carried off by a black man is unendurable.
Ultimately, Iago's attempt to discredit Othello on the basis of his marriage falls through. Therefore, Iago uses the marriage itself as his weapon, repeatedly stereotyping Othello racially. At the moment he and Roderigo hatch their plot to ruin the marriage, Iago explains what "they" are really like and how he, artful Venetian that he is, can control them: "These Moors are changeable in their wills . . . the food that to him now is as luscious as locusts, shall be to him shortly as bitter as coloquintida" (I.iii.350-2). The method he uses is jealousy, something with which Iago seems all too familiar.
There is also some evidence that Iago is taking sexual revenge, for he appears to believe that Othello may have cuckolded him: "I hate the Moor;/And it is thought abroad, that 'twixt my sheets/He has done my office: I know not if't be true;/But I, for mere suspicion in that kind,/Will do as if for surety" (I.iii.392-4). Iago suspects "the lusty Moor" of leaping into his seat; he will give him wife for wife (II.i). In other words, Iago's own jealousy, which is in no small part racially based, predisposes him to destroying Othello. The fact that he has been overlooked in promotion by someone he despises because of his race encourages his anger, and the fact that he has an opportunity to destroy Othello's marriage to a white woman sweetens the revenge. "I will gyve thee in thine own courtship," he decides, and the jealousy plot begins to take shape.
The cynical opportunism is all the more apparent for the reason that in any case Iago is pretty much indifferent to women...