How we cite our quotes:
Look to her, Moor, if thou hast eyes to see:
She has deceived her father, and may thee. (1.3.10)
Brabantio perpetuates a pretty unfair stereotype of young women in these lines – he suggests that since Desdemona has "deceived her father" by running off to elope with Othello, she'll probably "deceive" her new husband too. The idea is that an unruly daughter will make an unruly and promiscuous wife. Compare this to 3.3.17, below.
She did deceive her father, marrying you;
And so she did. (3.3.18)
When Iago wants to make Othello suspect Desdemona's been unfaithful, he suggests a woman who disobeys and "deceive[s] her father is likely to screw around on her husband. Othello's response implies that he feels the same way. Instead of seeing Desdemona's decision to elope with Othello (despite her father's disapproval) as a sign of his wife's loyalty to him, Othello sees Desdemona's willingness to elope as a prelude to her infidelity. It seems that Othello's sexist assumptions leave him pretty vulnerable to Iago's plotting.
Look to your wife; observe her well with Cassio;
Wear your eye thus, not jealous nor secure:
I would not have your free and noble nature,
Out of self-bounty, be abused; look to't:
I know our country disposition well;
In Venice they do let heaven see the pranks
They dare not show their husbands; their best conscience
Is not to leave't undone, but keep't unknown. (3.3.17)
Iago claims that Venetian women can't be trusted because they all deceive their husbands with their secret "pranks." This seems to be the dominant attitude in the play, wouldn't you say? Just about every male character in the play assumes that women are promiscuous and disloyal. Perhaps this is the reason why Iago is able to manipulate Othello into believing that Desdemona is unfaithful.
History Snack: In Elizabethan England, Venice was infamous for its courtesans (prostitutes). When Elizabethans thought about Venice, they often imagined it to be a city full of promiscuous women. Check out what Thomas Coryat has to say in his account of his travels to Venice:
[t]he name of a Courtezan of Venice is famoused over all Christendome […] The woman that professeth this trade is called in the Italian tongue Cotezana, which word is derived from the Italian word cortesia that signifieth courtesie. Because these kinds of women are said to receive courtesies of their favorites […] As for the number of these Venetian courtesans it is very great. For it is thought there are of them in the whole city and other adjacent places, as Murano, Malamocco, etc. at the least twenty thousand, whereof many are esteemed so loose that they are said to open their quivers to every arrow, a most ungodly thing without doubt that there should be tolleration of such licentious wantons in so glorious, so potent, so renowned a city." (Coryat's Crudities, 1611)