Officially, there were no interracial marriages under Apartheid, but in reality, the picture was much more complicated.
Apartheid rested on the separation of races at every level, and preventing interracial sexual relations was an essential piece of that. The Prohibition of Mixed Marriages Act from 1949 explicitly prevented white people from marrying people of other races, and the Immorality Acts prevented people of different races from having extra-marital sexual relations.
Moreover, the 1950 Group Areas Act prevented people of different races from living in the same neighborhoods, let alone the same house.
Yet despite all of this, there were some interracial marriages, though the law did not see them as interracial, and there were other couples who broke the Immorality Acts and were often jailed or fined for it.
The Prohibition of Mixed Marriages Act was one of the first steps in setting up Apartheid, but the law only criminalized the solemnization of mixed marriages not the marriages themselves. There were a small number of interracial marriages prior to that law, and while there was not much media coverage given to these people during Apartheid, their marriages were not automatically annulled.
Secondly, the law against mixed marriages did not apply to non-white people, and there were proportionally more interracial marriages between people classified as “native” (or African) and “Coloured” or Indian.
But, while there were in effect "mixed" marriages, the law did not see them as interracial. Racial classification under Apartheid was based not on biology, but on social perception and one’s association.
A woman who married a man of another race was, henceforth, classified as being of his race. Her choice of husband defined her race.
The exception to this was if a white man married a woman of another race. Then he took on her race. His choice had marked him, in the eyes of white Apartheid South Africa, as non-white. Thus, the law did not see these as interracial marriages, but there were marriages between