Well before "defending" any beliefs, I would submit that anyone who wants to change the status quo carrys the obligation to show why it should change; the status quo should not be guilty unless proven innocent. So other than gays wanting to have this right as arbitrary choice, what are the reasons?
For one, who or what decides gay marriage IS a right? If it is a right for gays, then why not brothers? Why not be able to marry a tree? The arguments I've heard offered go basically along the line of this:
gay = human, humans have rights, => gays have rights.
Well this is true when it comes to basic human rights, but not all rights are equal. Everyone in America has a Constitutional right to bear arms; but those rights are not equal among the law-abiding citizen and the felon; the sane, rational person and the person with a mental illness; etc. All enjoy the basic right to life, yet other rights are sometimes restricted for reasons such as safety, security, or betterment of society.
So the question is, is marriage some kind of basic human right, open to anyone and everything, or does society act justly when it carves out certain parameters within which marriage must operate? I say the onus is on the homosexual to demonstrate that is the case, and thus far they have not. All they have shown us is weak analogies and emotional appeal in an attempt to get what they want, which isn't marriage but a more robust legitimization of their "lifestyle."
Other than a piece of paper, please demonstrate what fundamental difference there is between a gay couple who enjoy all the same work/visitation/insurance/financial benefits without being married, and a couple who is married. It doesn't change anything, so why is there such an effort to seek this non-difference if it is not to lend credibility to a relationship that has never been recognized in all of history as legitimate? What changed that now makes it suddenly legitimate only by force of a legal decree?
In order for any society to work, it must have authoritative power invested in something from which standards may be derived. In the US, the nation was founded on the morals of Christian belief of that time. In that period, it was the commonly held belief that laws were of two types: divine, and man-made, and the understanding was that no man-made law could ever supersede a divine law. Example: There is a divine law against murder, thus no law could be made that sanctions murder, for it would conflict with a divine law and would be WRONG.
Lately, with the push on deconstuctionist and relativist ideas, many in society today choose to ignore any sort of divine law, substituting it for man-made laws, and where something must serve as that higher source, they will call it something like a "universal law," murder being the prime example, as though suddenly one day a huge world council sat down and came up with the idea that murder is wrong. Well it didn't happen that way, and history proves anyone who might think it did wrong. The point is, in the case of gays, they will vigorously resist any idea of a higher ideal from which laws come because that same source condemns their very relationships.
So without the ability to appeal to a higher power other than man, they appeal to man and his arbitrary decision making and cast themselves as unjustly singled out for persecution and bias because they can't have what they desire. However societies cannot work in such a system where the highest common ideal is the lowest common factor, no matter how low. Diversity has limits, for without it, you cannot justify any reason to stop dad from marrying all his daughters, his eldest son from marrying Spot the dog or the family goldfish. Perfect diversity is the absence of any limits whatsoever. That means there cannot even be such a concept as truth, unless and only unless truth can be anything and everything at once. Everyone is right, no matter their choice.
We would never accept a student who demands an A on his paper that cited the answer to 2 + 5 = 11, yet that means we must draw a line, we must of necessity make a decision and we must discriminate, or make distinction between right and wrong, and everyone knows 2 + 5 = 11 is wrong no matter how much the student wants it to be right, no matter how persecuted he feels at the hand of those who answered "7."
Gays send us these messages without much proof to back it up.
1. They can't help who they are, no choice is involved
2. They aren't treated equally because they can't have a marriage.
3. No one may criticize their lifestyle, while they may criticize, even demonize anyone else's beliefs and often do.
So in short, we have a dubious starting position (the 3 above), we have no compelling reasons offered beyond "we want it," and we have a clear attack on a society's justness in being able to draw a line or impose a limit. If I can't deny anyone such as a gay couple, because it is a RIGHT, then it's also a right to have a kangaroo for a spouse and to whom will you appeal to deny that that cannot be appealed to in the matter of gay marriage?