One doesn't to get married to reproduce---theoretically we could have no marriage records for our ancestors. Fortunately, most of the time we do. A recent article posted to our website discusses the main types of marriage records and ways to search for them:
"I know they were married, but I cannot find it. They had to get married; after all they had children. Well . . . not necessarily. Usually a marriage precedes the children. It is finding the record of that marriage that sometimes creates headaches for the genealogist. "
The rest of the article is in our article archives.