Just looking at marriage as a religious institution, why is the government involved at all? Baptism is another religious institution that has some controversies around its definition/restrictions. A Catholic believes that sprinkling water on an infant is baptism. A Baptist believes it's only valid if performed by immersion on a believing adult. Should the government get involved in declaring which baptism is valid? I know of no American that believes it should do so. Similarly, even within Christianity, there are differing opinions on marriage. The Presbyterian church down the street may have a heterosexual marriage ceremony, while the Episcopal church right next to it performs a homosexual marriage ceremony on the same day. Why should the government be involved in that religious debate? Why should this be different from baptism?
Well, the government is involved in the recognition of marriage simply because it has practical legal implications. It helps with how taxes are filed/collected. If you have an accident and go into a coma, it has implications on who is allowed to visit you since you are unable to approve people at the time. If you adopt a child, it has implications on what happens to that child if you die. It has practical implications on sharing a health insurance plan. The DOMA case the Supreme Court will hear tomorrow started with the issue of the estate tax and marriage: two women were married in Canada, moved to America, then when one of them died the other was taxed on "the inheritance" since we didn't recognize their marriage and therefore their joint finances. Here's a list of some more legal implications of marriage.
Notice that all the above legal implications of marriage are useful (and important for the well-being of those involved), and that they are just as useful when applied to a life-long committed gay couple as to a life-long committed straight couple. And this usefulness is the reason the government recognizes marriage but not other religious institutions like baptism. If you think marriage is recognized by the government so that it can enforce the will of God, why not also have it declare which type of baptism is valid? The government should recognize a gay marriage performed at an Episcopal church for the same reason it recognizes a straight marriage from a Presbyterian church. Not because it's declaring which one is right in a religious sense, but because it has practical legal implications.
Consider some other ways the traditional Christian understanding of marriage differs from how the government treats it today:
- Not requiring a religious ceremony. A couple can get married (in the eyes of the law) by just signing a document at a courthouse. Many Christians believe, particularly if that couple is also Christian, that they must have their religious wedding ceremony before they are really married. But I don't know of anyone that believes the government should add that restriction... because the religious ceremony is not necessary for the practical legal uses of marriage recognition.
- Divorce. The Bible clearly states that divorce (with a few exceptions) is wrong. For that reason, there actually used to be (maybe still are in some places?) laws to try to enforce that. Not allowing "no-fault divorces" was used to have the government prohibit a couple from divorcing unless they could prove there were circumstances that would make it morally acceptable. Nowadays, I don't know any Christians that believe we should use the government to try to force people to not get divorced.
- Re-marriage. Some churches have restrictions on what remarriages they will accept. In the Catholic Church, their previous marriage must first get an official "declaration of invalidity". In the Orthodox Church, a person can only have up to 3 marriages - any more after that will not be recognized. But I don't know of any Catholics or Orthodox Christians who believe the government should also use those restrictions.
- Inter-faith marriage. The Bible says Christians should not marry non-Christians. Therefore, many churches will not perform or recognize inter-faith marriages. But I don't know of anyone who thinks that restriction should be part of how the government chooses which marriages to recognize.
So no matter your personal/religious views on homosexuality, I still know of no reason to oppose legal recognition of gay marriage. The question of which marriages the government should recognize and which marriages your personal church should recognize are two different issues that exist for different reasons.