You will have to research South African immigration law with a lawyer or forum devoted to that subject.
In terms of U.S. law, as long as you meet the eligibility requirement of the state in which you marry, you can marry in the U.S. as a tourist. Some states are popular locations for destination weddings, and make it very easy for tourists to obtain marriage licenses. Each state has its own laws, so you or your fiancée will need to research the requirements for the state in which you intend to marry.

