Can We "Privatize Marriage"?
Creating a"family partnership" under federal tax laws, which allows you to divide business income among family members.
Receiving Social Security, Medicare, and disability benefits for spouses.
Receiving veterans' and military benefits for spouses, such as those for education, medical care, or special loans. Receiving public assistance benefits.
Consenting to after-death examinations and procedures.
Filing for stepparent or joint adoption.
Receiving equitable division of property if you divorce.
Receiving spousal or child support, child custody, and visitation if you divorce.
Suing a third person for wrongful death of your spouse and loss of consortium (loss of intimacy).
Claiming the marital communications privilege, which means a court can’t force you to disclose the contents of confidential communications between you and your spouse during your marriage.
Note that some of these would be around even in a nightwatchman state, so the libertarian answer,"get the state out of marriage," doesn't settle the issue. Tax reform and privatization would get around the family partnership and survivors' benefits issues. But there would still be civil and criminal courts and questions of child custody even in a minarchist utopia, so the state by necessity would have to decide what counts as a marriage for those purposes.