HUNDREDS OF GUESTS--INcluding Nelson Mandela--witnessed the elaborate, high-society wedding earlier this year of Debora Patta, a prominent white South African journalist, and her black sweetheart, Mweli Mzizi. So it came as quite a shock when Mzizi received a letter from the Home Affairs Department informing him that he was not legally married. According to an obscure 1927 statute, all black men are required to fill out a form prior to their nuptials, swearing they aren't already hitched before they say "I do." Explained one Home Affairs spokesperson: "It's just to protect the women." Few South Africans are aware of the law. Which means that many of the country's marriages are probably illegal in the eyes of the bureaucracy.