Correction: Isabella is not unfailingly virtuous.
Claudio asks Lucio to acquaint Isabella with his fate that she might persuade Angelo for, "in her youth/There is a prone and speechless dialect/Such as move men; beside, she hath prosperous art/When she will play with reason and discourse,/And well she can persuade" [1.2.179-83]. Though Claudio's last remark makes allusion of her astute ability to bend words, it is also used in juxtaposition with her "speechless dialect/Such as move men," referring to sex; Claudio is inferring that Isabella is sexually talented and her level of competence can move a man as hardened and mad as Angelo. This idea is followed in Act 2.2 where Angelo and Isabella converse with the assistance of Lucio. This scene starts slowly then rises to a climax that leaves Angelo flustered, frustrated, and undone. The rhythm is intentionally similar to the rhythm of sex but is transmitted through the Aristotiliean use of language; moreover, it is intriguing to recognize how this process was disturbingly assisted by Lucio as he coached Isabella into the act
[SIDE NOTE: The rhythmic effect is not uncommon in the Shakespearean canon, in fact, Shakespeare often made use of this device to communicate the idea of sex; one particularly familiar instance can be found in Juliet's reversed epithalamion in _Romeo and Juliet_.]
Furthermore, the Duke does not ask Isabella to marry her, he subjects her to marriage and she is not given the opportunity to reply, it is assumed that she will marry him. This is important to note as it propels the theme of gender politics in this play. As you read Measure for Measure (as I hope you will) keep in mind the thread of gender politics, keep also in mind how the imposition of maxims condemning natural desires result in objectifying women; this is Shakespeare's commentary on the hypocrisies of certain social structures of his time and their destructive effects on the oppressor and the oppressed. Shakespeare was especially known for making such arguments. His most popular device is perhaps his reversal of the sonnet conventions that would make women into voiceless and chaste love objects, forever lacking in sexual willingness, this is particularly evident in _Romeo and Juliet_ where Juliet's identity is rendered through her sexual desires and through the use of sonnet structure.