There is an additional penalty for those suitors who choose the wrong casket. They must, as Portia says, swear an oath to remain a bachelor for the rest of their days:

Or swear, before you choose,—if you choose wrong, 
Never to speak to lady afterward 
In way of marriage; therefore be advis'd.

So not only will they be denied the chance to marry the beautiful Portia, and benefit from her considerable dowry, but they will be condemned to bachelorhood for the rest of their lives. So the stakes are extremely high in making their decision, which is exactly what Portia's father wanted. He was determined that only worthy men should court his daughter and benefit from his legacy.