Desdemona was the daughter of Brabantio. Desdemona felt deeply attracted to othello. Brabantio felt hurt when he found out that Desdemona went to go elope with Othello and he had no notice of this. Brabantio doesn't like the fact that she went off with the moor, in fact he thinks that its outrageous. Even though Othello and Brabantio were close at one point, their relationship seems to be rocky now. In the text it states that, “Look to her, Moor, if thou hast eyes to see. she has deceived her father, and may thee” ( I, III, 334-335). Brabantio is saying that if Desdemona has felt the need to deceived him and not conclude him in this conquest, she may as well do the same to the moor. Brabantio feels as though Desdemona was not loyal to him and that she left him on his own.
With all the commotion that has been going on with Desdemona, Othello, Iago, and Cassio, its the most absurd to say that Desdemona was messing around with Cassio. With the constant bugs being put into his ear, Othello had no choice but to take what was said to heart and believe everything. He doubted the loyalty