How we cite our quotes: Citations follow this format: (Act.Scene.Line). Line numbers correspond to the Norton edition.
Quote #4
IAGO
Now, I do love her too,
Not out of absolute lust (though peradventure
I stand accountant for as great a sin)
But partly led to diet my revenge,
For that I do suspect the lusty Moor
Hath leaped into my seat—the thought whereof
Doth, like a poisonous mineral, gnaw my inwards,
And nothing can or shall content my soul
Till I am even'd with him, wife for wife,
Or failing so, yet that I put the Moor
At least into a jealousy so strong
That judgment cannot cure. Which thing to do,
If this poor trash of Venice, whom I trace
For his quick hunting, stand the putting on,
I'll have our Michael Cassio on the hip,
Abuse him to the Moor in the rank garb
For I fear Cassio with my nightcap too, (2.1.313-329)
Iago is completely obsessed with infidelity. Earlier, we heard him say that he suspects Othello has slept with Emilia (a sentiment he repeats in this passage). Not only that, he also says he "fear[s]" that even Cassio is sleeping with his wife. What's Iago going to do about it? Why, he's going to try to sleep with Desdemona, which will allow him to get even with the "lusty Moor." If he can't do that, he wants to make Othello believe that Desdemona is screwing around with Cassio.
Quote #5
IAGO
She did deceive her father, marrying you,
[…]
OTHELLO
And so she did. (3.3.238, 241)
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.
Quote #6
OTHELLO
O curse of marriage,
That we can call these delicate creatures ours
And not their appetites! I had rather be a toad
And live upon the vapor of a dungeon
Than keep a corner in the thing I love
For others' uses. Yet 'tis the plague of great ones;
Prerogatived are they less than the base.
'Tis destiny unshunnable, like death.
Even then this forked plague is fated to us
When we do quicken. (3.3.309-318)
Get your highlighter out because this is important. When Othello is convinced (by Iago) that Desdemona has cheated on him, he reveals something pretty interesting about himself. It seems that Othello believes all men, both "great" and "base," are "destin[ed]" to be cuckolds. FYI: A "cuckold" is a man whose been cheated on by his wife – cuckolds are commonly associated with horns, which is why Othello refers to cuckoldry as a "forked plague" that men suffer from.
So, if Othello believes that all men are destined, from the moment of their birth, to be cheated on by their wives, then this helps to explain why Othello is so easily convinced that Desdemona has been unfaithful, despite the fact that Iago never actually shows Othello any real evidence.