My thought:

Buy an "official" family Bible and have it blessed by your favorite pastor. Write your names, birthdates and something to the fact "in the eyes of God Our Father as our witness and partner, we do hereby claim each other as husband and wife, to have and to hold, from this day (whatever date you claim) forward." Sign it. Date it. Get others to sign and date as witnesses, if available. Live by it as an agreement / contract with God.

Might want to reference your parents' names, birthdates and marriage dates, as well. Have your kids put their same info in that Bible, as well. Continue through the years.

Read from it daily. Pray with it daily. Make it the centerpoint of your home, your life and your family heirs and their lives.

If you want to make it more legally binding in society, photocopy the page and submit it for a simple newspaper article declaring it for public record.

I imagine this is pretty much what has been done for thousands of years before governments intervened to the extent they have, lately.

Again, my thought. Or, you could ask the Amish.