Measure for Measure is a play obsessed with death, both physical and spiritual. On the subject of mortality, Claudio delivers one of the greatest speeches in all of Shakespeare (one that many critics think rivals Hamlet's famous "To be, or not to be" soliloquy). For many characters, death goes hand in hand with sex and marriage. At other times, the play's view of mortality is more flippant than serious, as when Barnardine insists he's too hungover to be executed and when Pompey declares that a good hanging is the best cure for a hangover.
Barnardine's blunt and comical refusal to be put to death draws out attention to the Duke's hypocrisy and immorality.
For Isabella, sex is an act that's worse than physical death.