Marriage these days is either a romantic gesture or a financial arrangement anyhow, maybe with child-rearing as a distant third purpose (secondary to the first two usually). Viewed from this perspective, there's nothing particularly sacred or meaningful beyond that. Why not allow gay marriage? While I may think that homosexuality is strange, or can't wrap my head around it, I still won't limit other people's rights or capability to do what they think is best.
Meanwhile, from the perspective of religious or moral reservation, where homosexuality and gay marriage are viewed as sinful/unnatural/wrong, and where marriage is considered a sacred rite, then to that I say: there's a reason why church and state are separate. The purpose of government is to keep order, and to keep society running smoothly. Government is not there to impose morality. If the church disagrees, or if individuals with their moral standards disagree, they can simply refuse to honor or recognize the marriage.
It is not the place of the government to decide whether gay marriage is legitimate or not. To the government, romantic ideals and moral ideals are secondary. Marriage to the government is a matter of tax laws and who has rights over the children, pets, and property. And... to decide how to partition the property when a couple decides to have a divorce.
Hell, according to the Catholic church, my parents aren't married. But they still file their taxes jointly, and I imagine if one of my parents was unconscious, the other would have the power to decide what kind of medical care and legal matters are necessary. Screw the Catholic Church if they don't believe in divorce, and believe that my father is still married to his first wife. He hasn't even seen the first wife in something like 40 years. But... the Catholic Church has every right to believe what they want to, and decide whether they want to recognize it or not.
(Fun story, that. It involves the husband of the second wife wanting to convert to Catholicism and have his marriage recognized as legitimate. If anyone ever calls me a bastard, I can truthfully respond with "Actually, according to the Church, I am a bastard.")
Though, when it comes down to it, I believe the government has allowed "civil unions" between gays, but not "marriage". I have to say I don't know what the difference is, legally. Google to the rescue!
http://lesbianlife.about.com/cs/wedding/a/unionvmarriage.htm
I would be in favor of expanding "civil unions" to a Federal level, and including most of the rights and privileges of marriage into it. Or, for that matter, rename "marriage" as "civil union" in order to detach any cultural and idealistic meanings from the legally binding contract between two people. "Marriage licence" becomes "Civil Union Licence" and all the legal wording gets reworked to be politically correct and orientation neutral. I believe most of the opposition to "gay marriage" is semantic anyhow.