More on gay marriage: I don't have much time to be drawn into debates (in fact, I'm on the road as we speak) but at times the goading of readers makes me want to bite, and unlike some, I think issues connected to the institution of marriage are important.
There are many available arguments against gay marriage, the most obvious one being that the majority doesn't want it.
But allow me to take another tact--I'll steal from Aristotle. We can infer the purpose of something by its design. By examining the construction of a hammer, we can see that it is meant to hit things. (And here I'll use "penis" or other related words as a shorthand for the whole sexual reproduction system). By examining a man's parts, it is clear that their purpose is to impregnate some object, and we can see from studying the design of a woman that she is that object.
Now, clever humans manipulate sex to get pleasure without reproduction, but a penis pointed at a woman is likely to achieve its intended purpose sooner or later. One thing that ensures that the penis will not ever get around to serving its purpose is if the object of arousal is another man. It's, for example, like erectile dysfunction. As long as it persists, the purpose of a man's parts is thwarted. So same-sex sexual attraction can be seen as a dysfunction or a chronic health condition--probably one with no cure at the moment.
You might respond that a disability doesn't deprive one of his rights, like voting for example. But marriage is not a right. People can set up house with whomever they want. Marriage is the state recognizing a particular union. The government is putting its stamp of approval on the arrangement. It is holding up the relationship and saying, we like this and want to encourage it.
Throughout history, society via the law has venerated marriage for a variety of reasons, but it makes no sense to enshrine a relationship based on a disability. That would be like the deaf community that wants the government to put its stamp of approval on deafness, and to declare that it is just as good as hearing, and it should be venerated and encouraged as much as hearing.