What is the problem between Portia and the Prince of Morocco?

The Prince, one of Portia's suitor, wants to marry her, but she has no choice in who she marries. He meets with Portia and tells her that he is often considered very handsome on account of his black skin. She tells him that unfortunately she does not have the right to choose the man who will marry her. Instead, her father created three caskets from among which each suitor must choose. Portia warns the Prince that if he chooses the wrong casket, he must swear to never propose marriage to a woman afterwards. The Prince of Morocco agrees to this condition and joins Portia for dinner before attempting to choose.