This night's the time
That I should do what I abhor to name,
Or else thou diest tomorrow.
Thou shalt not do 't.
O, were it but my life,
I'd throw it down for your deliverance
As frankly as a pin. (3.1.113-119)
For Isabella, the thought of having sex is so horrifying that she can't even bring herself to say the word out loud. When she declares that she'd gladly sacrifice her life for Claudio, it becomes clear that, for Isabella, sex is worse than death.
This idea surfaces repeatedly throughout the play. Elsewhere, Isabella says, "Better it were a brother died at once / Than that a sister, by redeeming him, / Should die forever" (2.4.106-108), meaning that Isabella thinks she'll go to hell if she hooks up with Angelo (2.4). For more on this, check out "Quotes: Mortality."