Sex is a big deal in this dark play and it's often associated with death, decay, and corruption. In Vienna, brothels are a dime a dozen, the spread of venereal disease is rampant, and the numbers of illegitimate children are on the rise (social problems that also plagued Shakespeare's London). Despite the fact that prostitution and fornication are illegal, it often seems like the entire city of Vienna is engaged in illicit sexual activity. While the play acknowledges that illegitimate children and the spread of STDs are pressing social concerns, Measure for Measure also questions whether or not the government should be involved in regulating sexuality.
In Measure for Measure, sex is nearly synonymous for death.
Part of what appeals to Isabella about becoming a nun is that life in a convent will offer shelter from Vienna's rampant, hyper-sexuality.