The posts above have articulated a number of arguments in favor of the official recognition of gay marriage and I agree.
Equal treatment under the law is a right in Western countries, and in the letter of the law in most countries around the world. Why this equality would not apply to an institution based on love makes very little sense to me.
Whether you agree or disagree with gay marriage, you still have to consider the amount of control you want the government to have. A person who disagrees with gay marriage might not consider the repercussions of a law against it. They will be more likely to simply agree without considering the long term consequences. What happens years later when the government decides to create a law which bans something else? What happens when they want to control an aspect of your life that you don't agree with? The precedent has been set for them to have this level of control. This is the part that concerns me. Gay marriage is a personal and religious issue. I don't think we should have law which dictate and govern this aspect of our lives.
I heartily agree with frizzyperm's statement on this one. The Fourteenth Amendment to the Constitution states that no state can deprive a United States Citizen of the equal protection of the law. This provision was used to end (or at least work toward the end) of discrimination based on race. Discrimination in any form--including denial of the right to marry--is a violation of the Equal Protection clause. Only the most ignorant and blindly bigoted individuals still argue that a person's sexual orientation is a "choice." Since gay people are simply oriented differently from the mainstream crowd, I believe they have an undeniable constitutional right to do anything that anyone else can do, including marry.
I don't think this is going to be an issue for much longer. It's going to happen. The tide of human opinion has turned in this matter, and it isn't going to turn the other way, I don't think.
I believe society, legally at least, is going to become fully accepting of the gay movement. Of course there will be "pockets of resistance."
Many people argue that allowing gay marriage will help to undermine the idea that monogamous, permanent, heterosexual marriage is the ideal family situation. The most enlightened versions of this argument acknowledge that things like allowing divorce undermine this ideal much more than gay marriage. But they argue that everything that does anything to undermine this ideal is a bad thing.
There is no constitutional reason to prevent gay marriage. In fact, The Constitution demands that gay marriage be allowed, because we believe in Liberty and Equality for all.