I’ve mentioned this tip before, but it’s one that is good to remember.
What records or sources could directly state or indirectly suggest the event or relationship I would like to document? There may not be a birth record, but there may be other records that state the date and place of birth or that state the parent-child relationship. The same is true for a marriage. There may be records that document the marriage, but are not actually marriage records.
A birth may be documented on a death certificate, a marriage record, an estate settlement or other documents. The same is true of a marriage. Of course those records may provide statements that are not primary information. That analysis is just a part of the research process. It may be that those sources provide clues to help you find original sources that do provide primary information.