In the opening scene of the play, there are a number of examples of racial prejudice that come out as Iago presents to Brabantio the image of Othello making love to his daughter.
In the society of Venice, Othello is revered as a general but his blackness as a Moor is still a mark of difference, considered to be someone apart from the noble society that is acceptable to people like Brabantio and his daughter. It is completely accepted that he leads their armies in battle, accomplished general that he is, but the idea of him wooing one of their fair and noble daughters is repellant and shocking.
As Iago brings up the image of the black ram "tupping [the] white ewe," he is portraying their sexual act as an animalistic one, making sure to horrify Brabantio with the thought of an animal mating with his dear sweet Desdemona.
Though it will become apparent that she welcomed Othello's affections and played as large a role as he in their betrothal, the idea of it is very difficult for Brabantio to swallow and leads him to object strenuously to the Duke and request justice be done upon Othello.