I think everyone should have someone to share a life with – or at least the civil burdens and benefits thereof. What I would really like is a "Ruth and Naomi Law." Each person could designate one other person for all those civil purposes that you need to share with others – to make medical decisions, to visit you in the hospital, to inherit your stuff. For married people that designated Other would be your spouse. Unmarried people, though, could designate one person, too. As far as the law was concerned, this need not be a marriage-like relationship; the state would be quite uninterested in whether sex was involved. Such a law would also allow same-sex couples to share most of the civil burdens and benefits that married people share, though the status would be broader than either marriage or domestic partner unions.
It is more likely, though, that our governments would adopt civil unions, a much narrower "marriage-lite" status. This would accomplish nearly all of what proponents of gay marriage want.
I think a free society should offer civil unions to homosexual citizens who want them. Following the experience of the Scandinavian countries with civil unions, I don't think there would be a big rush to them once the first excitement died down. Denmark, which has the longest experience with civil unions, has seen only about a tenth of one percent of their population take advantage of the opportunity. Some people want gay marriage or the nearest approximation, but the numbers are not large – or stable. Even some of the most celebrated same-sex unions in this country, such as the test-case couples who won the right to marry in New Hampshire and Massachusetts, have already split up.
Moreover, in countries with civil unions, more heterosexual couples use them than homosexual. Why they don't just go ahead and get married is somewhat mysterious. Still, I think they can also be accommodated in a free society.
Robby George has mounted a powerful natural law argument that the law should only recognize marriage of a man and a woman because "the law is a teacher." The law, he argues, should not only reflect social conditions, it should shape them. I can almost go with him here, but I think the line that he draws between legal marriage of a sterile heterosexual couple and an illegal union of two homosexuals is just too fine, or two dependent on extra-constitutional metaphysics, to really fly in America.
Some think that civil unions will undermine marriage. I think this is true. I do not think, though, that they would undermine marriage very much – not nearly as much, for example, as no-fault divorce laws have. Even so, I agree that there is some injustice in married people enjoying some benefits that homosexuals cannot enjoy. Righting this injustice by offering the civil union status is worth some erosion of marriage – especially since, in practice, not very many homosexual couples are likely to actually make use of it.
The Presbyterian Church, and other mainline churches, have long supported civil rights for homosexuals, including civil unions. There is no contradiction in saying, as nearly all such churches do, that homosexual practice is a sin and cannot be approved in the church, yet at the same time supporting homosexual civil rights. The church has one constitution, and civil society has another, more limited constitution. The church follows its own law in church matters, such as whom it will ordain and whom it will join together in marriage. The church also witnesses to society, and supports a broad accommodation of all kinds of freedoms for citizens under the civil laws.
Civil unions of same-sex couples could form a good-enough institution in society and, with more caveats, in the church. And good enough is good enough.