I don't really get why so many people have such a problem with legalizing gay marriage. I get the "yuck" factor but I don't see it as an inherent evil nor do I understand how it will destroy civilization. Call me an optimist but I don't think Western Civilization is really all that fragile. Maybe that is just because I am not religious. I don't know.
Stripped of its cultural baggage, the civil institution of marriage is a contract -- mostly about the disposition of property. You can write that contract yourself -- it's called a pre-nuptial agreement -- or you can accept the default contract written by your state legislators. In the former case the property arrangements are usually pretty obvious. They are less so in the latter case but they are still there.
A large part of the problem arises because the state gets to define what is and is not a "marriage" and can even change the term of your agreement after the fact and without your consent. It then takes that definition, licenses it and hands out free shit based on it. The ideal would be for the state to get out of the business of defining "marriage" and only become involved when there is a contractual dispute. Once the government is only concerned with the civil aspects of the institution, the churches, synagogues, mosques, etc can perform the rituals according to their own customs.
The reality, of course, is that the individual states are not likely to give up control of marriage anytime soon. Until they do, they are, at the minimum, bound by the highest law of the land (AKA the Constitution and Amendments). Ideally they would operate from a sense of fairness towards all taxpayers but that may be too much to ask.
The Fourteenth Amendment promises equal protection under the laws to "[a]ll persons born or naturalized in the United States, and subject to the jurisdiction thereof". It is a broad guarantee that forbids class-based discrimination by government absent a damned good reason. For example, denying a driver’s license to anyone who is legally blind.
There is a common-law rule of interpretation that powers delegated to the state should be interpreted as narrowly as the language permits. Rights should be interpreted as broadly as possible. The presumption must always be in favor of a right against the actions of government, or immunity against the power of government. (Thanks to Jon Roland for explaining this in term a dumb 'ol engineer can understand).
Gay people pay taxes. According to some sources they pay more taxes per capita than straight people. They also pay these taxes under the same threat of punishment as everyone else. They have no more choice about their taxes than you or I do. That means money taken from gays helps fund the county courthouse and the the clerk's office. It also helps pay the salaries of clerks, judges and justices of the peace. So how the hell can a state deny gays access to services they are being forced to pay for?
Legally and, I contend, ethically it cannot. The government wants to take money from its citizen to pay for a service. Then that government wants to turn around and refuse to allow part of that citizenry access to the service. It has a hell of lot of explaining to do. In fact, the gun controllers are way ahead of the marriage controllers in that regard and you probably already know what I think of the former.
None of the above allows for ridiculous notions like forcing a baker to make a wedding cake for a gay couple. In fact, while the judge may have been interpreting the state law as written, that decision is an insult to basic decency. A private business does have a right to refuse service to anyone for any reason as long as it is the business owner's free choice.
A common objection to gay marriage is that legalizing it would somehow allow a person to marry his dog or his toaster or a child. That is a desperate and foolish argument. The truth is that neither a dog nor a toaster is sapient so neither can assent to a marriage contract. A child after a certain age may be sapient but, until he reaches the age of consent, cannot legally agree to a contract.