familysearch.org has a searchable index of the Catholic Church records of Guanajuato from 1519 to 1984 at https://www.familysearch.org/search/collection/1860831. I did not find a marriage of Aaron Medrano or Marcelina Campos there, so is it possible they did not have a church wedding? Or else the indexer mangled their names (which is always a distinct possibility with these databases).
familysearch.org also has the Civil Registration records for Guanajuato from 1862 to 1930 at https://www.familysearch.org/search/collection/1922031. This collection has not been indexed by them, but it has been indexed by ancestry.com (which, unlike familysearch.org, is a paysite). If you have an ancestry.com account, you can search the Guanajuato civil marriage records at https://www.ancestry.com/search/collections/60438/. Unfortunately, I have not found a marriage for Aaron Medrano and Marcelina Campos there, either (there was a Marcelina Campos of Jaripitiro who married a Leandro Arroyo in 1912, but I'm sure that's a different Marcelina Campos).
The 1930 census does state that both Aaron and Marcelina were born in Guanajuato, but perhaps they got married out-of-state.