What no one has yet mentioned is that Iago uses the Venetians' prejudice against Moors to pit them against Othello, a man otherwise admired for his military leadership. Othello inspires sexual anxiety, even Iago's own: "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," (II.1.379-382).
He uses Brabantio's concern, not only over his daughter's chastity, but also regarding her choice, to rouse up opposition. Shakespeare uses contrast in color and in species to illustrate the perceived importance of their difference ("an old black ram / is tupping your white ewe," I.1.88-89), as well as constructions of Moors -- who were Muslim -- as anti-Christian, even evil ("Or else the devil will make a grandsire of you," I.1.91).
To Roderigo, who hopes to win Desdemona from Othello, Iago describes Desdemona and Othello's love as passing, "a violent commencement in her." According to him, Moors are "changeable in their wills" and, thus, Othello would lose interest in Desdemona once her youth began to fade. He advises Roderigo to "put money in thy purse," to be prepared for the inevitability of Desdemona's abandonment. Here, Iago uses not only prejudices against Moors, but also prejudices against women, that they become less interesting with age, that they are most interested in money, to bend Roderigo to his will and, in an aside that parallels his advice, to "make my fool my purse."