And 'twere the cheaper way:Better it were a brother died at once,Than that a sister, by redeeming him,Should die for ever. (2.4.14)
Isabella not only wants to remain a virgin forever, but she also believes that having sex with Angelo would send her straight to hell. Is she right? Literary critic Katharine Eisaman Maus points out that "St. Augustine, the most influential Christian writer on sexual morality, insists that sin is a property of the will, not a physical state, [and] persons who are forced to perform sexual acts are blameless."
Still, that doesn't mean that Isabella should sleep with Angelo.