This is what I would do. Sometimes the marriage record will give the maiden names of the mother’s, so you don’t have to use “unknown.” The marriage record is the source for the marriage of the couple, and the source for the names of the parents. If the ages of the bride and groom are given, you can estimate when the parents were married. My genealogy software allows the use of words like about (abt), before (bef) or estimated (est), and others, with dates which makes estimated dates clear. The convention I use is that abt refers to a date based on another date (age 20 in the 1940 census means born abt 1920).