Hi, First I'm assuming they were married in San Francisco County which also includes the City of San Fran. Take a look at San Francisco Genealogy, sfgenealogy.com/sf/sfdata.htm .
You can also contact the Office of the Assessor-Recorder. Their contact info can be found at sfgov2.org/index.aspx?page=19 but ask for a copy of a non-certified or informational record. The certified copies have a lot of restrictions on them. Successful Searching!