OK, there are two competing interests here. The first is the desire to spread as broadly as possible the individual and social benefits of marriage. The second is to prevent the abuse of marriage by those simply seeking the the individual benefits without regard to the social benefits.
Who may engage in it? Any couple or group who choose to intermingle their lives and affairs in a relationship of mutual trust, excluding immediate family members. This caveat is not out of a concern about incest, per se, but instead to preserve the societal benefit of the interconnection of “strangers” into larger groupings.
What must they do to enter into it? Intermingle their lives and affairs. Examples are a shared primary residence, shared assumption of burden for the upkeep and debts of the other member(s) of the marriage, sharing of assets, etc. This is the key to the prevention of abuse.
How to terminate it? Divorce, either for fault or no-fault. A key caveat I would add is that if divorce is no-fault, and the marriage lasted less than a set limit of time (to be determined later), the divorcing parties must disgorge any benefit they received from the marriage, such as reduction in taxes, immigration status, etc. For protection of innocent parties, a divorcing party may challenge the no-fault status of the divorce.
Legal rights of marriage (property) - Shared ownership of all assets acquired during the marriage. Some level of ownership in assets acquired before the marriage - not sure what level, but pre-nups absolutely excluding pre-marriage assets should be invalid. Again, to prevent abuse and also to make parties think before entering into marriage. Right to void any contract affecting shared finances made by spouse(s) without approval. (May have to alter this to majority or super-majority for group marriages).
Legal rights of marriage (each other) - right to make any decision that would otherwise be made by spouse if spouse is incapable of making decision (except where the spouse has pre-stated decision). Right to veto and/or not assume responsibility for reproductive decisions made by spouse outside of the marriage. In group marriage situation, right to no-fault divorce without penalty if other members of group decide to reproduce without consent of objecting marriage member. Shared burden for support of children of spouses, both in marriage and pre-marriage (but see above for out-of-marriage children).
That’s about it for starters.
Sua