As we know, all Shakespearean comedies work their way toward endings that culminate in one or more marriages. This is also true of Measure for Measure, but, in this play, marriage doesn't necessarily bring about a happily ever after. For many characters, marriage is a form of punishment (literally and figuratively). For others, it's a fate worse than torture or death, making Measure one of the most cynical plays about the nature of marriage.
Lucio believes that marrying a prostitute is a fate worse than torture because being hitched to a promiscuous woman will compromise his masculinity.
Isabella's silence (after the Duke's marriage proposal) forces the audience to wonder whether or not marriage is the means to a happy ending.