The history of marriage is not one of love but of contract relationships, all the way back into the mists of antiquity.
Government involvement seems to have been initiated as (1) a means of generating revenue and (2) a means of controlling people (as in forbidding interracial couples from marrying or first cousins or adult/child marriages or group marriages). Sort of like those laws that allowed "eugenic sterilization" back in the early 20th Century.
Far better if it had been left a matter of private contract law. Let the parties set up their own terms and get government out of the way. Of course, the right to contract may be limited by age - as in "no one under the age of 18 can enter into a legally binding marriage contract." Leave it to marriage contract lawyers to work out terms as they do now with prenuptial agreements. I suppose there can be certain legal minimums in each contract and required terms like, "It is a breach of contract for one spouse to strike another." Penalties for such breaches can be spelled out in the contract.
The law governing all the benefits conferred on married couples/groups can be adjusted so that the benefits only flow if the parties involved produce a valid contract of marriage.
On the other hand, such law as there is on this topic should allow anyone to discriminate as they see fit against such marriage contract arrangements as they find repugnant. This ought to allow all parties maximum freedom to contract as they will without the fear of some government agency forcing them to act counter to their right to believe as they will. As one of my favorite science fiction works put it: F = IW
Or, as was written in Let’s Divorce Marriage from the Government
The best solution always has been the separation of marriage and state. If my priest decides to marry gay people, then my fellow parishioners would have every right to be upset about that based on their cultural traditions and understanding of Scripture. If your pastor wants to marry gay people, then it’s none of my business. The terms of marriage should be decided by religious and other private organizations, and the state shouldn’t intervene short of a compelling reason (i.e., marriage by force or with children).
Liberals were more open to this "separation" idea back when conservative pro-family types were ascendant. Now, some conservatives are understanding its merits as a more liberal view is ascendant. Conservatives should have listened when they had some bargaining power, but everyone wants to impose their values on others by using government.
Government neutrality -- or the closest we can get to it -- is the best way to ensure fairness and social peace on this and most other social issues. Marriage is too important of an institution to be dependent on the wiles of the state. Do we really care if the state validates our marriage licenses?