How we cite our quotes: (Act.Scene.Line)
Quote #1
Cardinal: Shall our blood,
The royal blood of Aragon and Castile,
Be thus attainted?
Ferdinand: Apply desperate physic!
We must not now use balsamum, but fire,
The smarting cupping glass, for that's the mean
To purge infected blood, such blood as hers. (2.5.21-26)
Having learned from Bosola that the Duchess has just had a baby, Ferdinand straight-up loses it. The Duchess's blood is his own, and he can't stand the idea of it mixing with that of a social inferior. Gasp! What's especially wacky here is that at this point Ferdinand has no idea who the Duchess has been making babies with. Heck, for all he knows it could be another aristocrat. To Ferdinand, it's as if anybody outside of his direct family has "tainted blood," and that real purity can only be maintained by the Duchess's total chastity.
Quote #2
I could kill her now
In you, or in myself, for I do think
It is some sin in us heaven doth revenge
By her. (2.5.64-66)
Ferdinand, having found out about the Duchess's pregnancies, is venting to the Cardinal. Just as he can't distinguish between the Duchess's blood and his own (see Quote 1), he also can't distinguish between her behavior and his own. To Ferdinand, the Duchess's taking a lover (he doesn't know she's married yet) isn't the independent choice of a free woman, it's an act that has immediate bearing on him. In other words, he thinks her sexual behavior not only affects him, but is also somehow happening because of him. Um, get over yourself, buddy.
Quote #3
Ferdinand: I am to bespeak
A husband for you.
Duchess: For me, sir, Pray, who is't?
Ferdinand: The great Count Malateste.
Duchess: Fie upon him!
A Count? He's a mere stick of sugar candy,
You may quite look through him. When I choose
A husband, I will marry for your honour. (3.1.38-44)
Ferdinand is pretending to offer up a husband for the Duchess, whom the Duchess immediately rejects as flimsy. Notice that, even when she's pretending to cooperate with Ferdinand's plans for her remarriage prospects, she talks about "choosing" a husband—he can line 'em up, but the selection has to be left to her. Also, notice that she says she'll remarry for Ferdinand's "honour"—what do you think she means by this? Is she referring to lineage or merit?