Lawful Advantage and Marriage

There would appear to be no purpose in a law that does not confer advantage, or disadvantage, on one or more parties the law relates to. It has to approve or disapprove of something as against something else. Naturally, it may be necessary for the law to define a social condition, but only for the purpose of other associated laws conferring advantage, or perhaps protecting the parties from disadvantage from others.
If adults are permitted to live together in any relationship that is mutually beneficial, assuming some broad definition of that benefit, that would presumably include a sexual relationship.
But what advantage or disadvantage could civil marriage confer on these same people. A tax or similar incentive would imply their relationship is better for society than another entirely undistinguishable relationship. They might indeed like the idea of 'marriage' as a treaty between themselves, but this may be private, with no possible need for society to be involved.
The same parties may wish to be married in church [etc] but this takes the contract out of general civil society into the realm of a particular society, to the principles of which they must subscribe, and which are subsidiary to that of general society. Marriage in church between other than heterosexual couples may therefore not be permitted. For if the church is not simply to be a tool of the state, or general society, it may promote principles that are not in conflict, but which confer narrower limits of right.
But at some point the possibility of children being procreated must arise. At this point society must be involved in order to protect the child. In this case, four parties are involved man, woman, child, and society [God]. A marriage contract is unavoidable. It is also very problematical how the full rights of the child can be respected, if there is doubt as to parentage and family descent. This having implications for the casual relationships of adults first considered.