I have a large number of couples for whom I have no marriage event, either because they had children but never married, or because I have no clue as to when or where the event occurred.
For that first category I created a new custom fact named "NOT MARRIED", assigned as a couple-specific fact. I added this so I could see it in the narrative reports. I give the fact a date if I feel the need to sort it, but do not use the date in the fact's sentence. If you like that idea, RMG has an excellent Help file, and they have very helpful forums on their own website.
Your great grandparents were not married, but they were a couple for the purposes of genealogy, as was stated before, they wouldn't be considered married by the program. I created my custom fact so I could tell the difference between those I knew to not be married and those about whom I wasn't sure.