In an offline genealogy program I use I enter the phrase "(not married)" in the marriage date field. The program gives me an error message that this is a "non-standard date" but it accepts it. This phrase prints in reports where the date would appear.
I only use this phrase when I know the couple was not married.
If I do not have a marriage date I either leave the date field blank, or may estimate a date. Estimating a date is precarious as there are sometimes unrecorded children, or the marriage happens after a child, or two (or more), is born. I've seen as many as four children being legitimized when a couple married.