not just of Roderigo's infatuation with Desdemona but of Roderigo's sense that Desdemona has betrayed her own kind.
It may be inferred that partly for Roderigo's benefit Iago uses coarse language to describe the elopement and the sexuality between Othello and Desdemona as forbidden (and offensive) fruit. He wants Roderigo to explain the details of the elopement, and he incites Roderigo to striking just the right tone by couching his own words in the terms of racial hatred. "Even now, now, very now," Iago tells Brabantio, "an old black ram / Is tupping your white ewe" (I.i.88-89). If the objection were merely that Othello is a gentleman of a certain age, the reference to black and white would not come into the equation. But Iago is deliberately inflaming the anger of suitor and father, with a view toward destroying Brabantio's hitherto good opinion of Othello. As the scene builds, so does Iago's coarseness. He conjures the image of Brabantio's "daughter 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. Bad enough she should have been carried off; to have it done by a black makes it horrible. This offense against comity, which is really a love match (at the beginning) illustrates the offense that blacks are presumed guilty of when they marry whites.
It is at this point that Iago allows Roderigo to take over. He describes Desdemona's elopement as "a gross revolt; / Tying her duty, beauty, wit and fortune / In an extravagant and wheeling stranger" (I.i.133-5). One senses that had Othello been white the elopement would not have been characterized by offense, for Brabantio's reaction is to describe the elopement as Desdemona's "treason of the blood" (I.i.170). Thus incited, Brabantio carries the feelin...