Like most libertarians, you haven't given this much thought at all.
Marriage is a private matter. Marriage does not depend in any way upon the approval or involvement with anyone else. Burglary, theft and murder all involve uninvited third parties.
Two people on a deserted island can enter into the marriage covenant with each other and with God without the involvement or knowledge of any government.
I have given far more thought than the vast majority of reactionaries on this forum. I would not have proposed it but for the fact that government now seems “hell-bent” to destroy marriage as God defined it.
Moreover, people who object to my proposal never seem to care about co-habitation in college dormitories, nor that over 72% of black children are born to a mother who is unmarried. They will contribute to the decline of marriage as government continues to smother it.