Resources include, but are not limited to property, food, shelter, sex and social interaction. All resource sharing outside of the couple can be done with the permission of both parties except when this action requires futher sharing of resources on an ongoing basis.
Example 1: Sharing sex might result in progeny that will need resrouces.
Example 2: Sharing of shelter can require sharing of resources as long as the shelter is shared.
Though the contract does not need to be administered by an outside entity it does need to be binding with consequences for breach. This might include limited sharing of resources.