For most religious people, marriage happens on three levels: legal, religious and metaphysical. Let’s take a look at these three and sort through the implications of the distinctions.
The simplest distinction here is the legal marriage. People of any religion or race can enter a legal marriage. It requires applying at a courthouse, and signing a contract. The contract of a legal marriage, and the laws surrounding it, establish ownership, next of kin, and tax filing status. These items are all the domain of the state, and thus the state provides this legal marriage contract.
Most people are familiar with religious marriage. In Christianity, marriage is viewed as sacred; it is an analogy for the relationship between the church and Christ, and for the relationships within the trinity. It is a covenant and, in some denominations, a sacrament. These items are all the domain of the church, and thus the church forms this covenant between two people.
There is yet a third distinction, that is very much related to the religious marriage. It is however more basic, and more inexorable. Jesus points out the metaphysical relationship between a married couple in the Sermon on the Mount. He says:
“It was said, ‘Whoever divorces his wife must give her a legal document.’ But I say to you that everyone who divorces his wife, except for immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
—Mt 5:31-32, ESV
The first part of this quote, Jesus is referencing the religious precedent as well as acknowledging the legal aspect of marriage. He indicates that the religious practice was to dissolve marriage through legal means. He then goes on to state that even if this divorce went through, the couple would still, in reality, be married. The permanency of marriage is inexorable no matter what the state or the religious institution have to say about it. Otherwise, how could marriage to a divorced person be adultery? It could not.
Preston Yancey states in his blog (and correctly so) that matters of the church are strictly not in the domain of the state. Therefore, he continues, the state should have no right to define marriage, much less redefine it.
The problem here is that legal and religious marriage have been conflated. The state cannot (and should never attempt) to define the sacred covenant of marriage. But ONLY the state can define the terms of the contract determining property rights, next of kin, and tax status. This is reciprocally outside the domain of the church.
And what about metaphysical marriage? There is nothing we can do to change it. Only God could alter the reality of marriage. If the state defines legal marriage to include two men or two women, not only does this not require the church to change its definition (whatever it may be for a given denomination) but it cannot even come close to having any effect on whether or not the legal marriage exists metaphysically.*
*Denominations disagree on the definition of marriage because they disagree about the metaphysics. Philosophically, it is possible that every single church has an incorrect understanding of the metaphysics, and yet the metaphysics are still unaffected. I strongly suspect this may well be the case.