Conservatives and Gay Marriage
When a conservative rails about the sanctity of marriage, protecting marriage, keeping marriage safe from gays, all a liberal hears is homophobia, but there's more to it than that.
Recall the foundations of morality? Dr. Haidt's work has elucidated further differences between liberals and conservatives, namely how they see society.
For liberals, the society is atomic, with individuals forming its base. Their associations and interactions form the larger groups, communities we see. This worldview perhaps reached its apotheosis in the American Revolution. The Constitution enshrines the rights of the individual, followed by the liberties that prevent action by the state and federal governments.
The conservative worldview is molecular, based not on the individual but on the family. The third pillar of morality is in-group loyalty, and there is no loyalty more binding than that of blood. This morality was extended to non-family in the bronze age ethic of sacred hospitality; someone invited into your home is family, thus neither group can betray the other.
For the conservative, all society is based upon blood loyalty writ large. When two people marry, they are uniting two families in an unbreakable blood oath. All families thus knit society together, with blood and marriage forming the warp and the weft. Without the ties of marriage uniting separate families, there would be nothing to hold society together and civilization would fall apart.
This answers the liberal question "How does two gays marrying hurt your marriage?" and also answers the conservative hatred of divorce. By weakening the institution of marriage, it weakens the weft of society. Divorce makes marriage a frivolous pursuit, entered into and exited with ease, turning all extra-familial relations into nothing more serious than friendship. The conservative fallacy of the slippery slope whereby gay marriage leads to polygamy or marriage to animals is the same argument. Conservatives feel homosexuality is a sin, a deliberate descent into wickedness. Enshrining gay marriage would be like giving an orgy or a drinking binge the same solemnity and weight as marriage, which is to say marriage would have the same solemnity as a binge; none at all.
So when a conservative rails on about the sanctity of marriage, he means it. A liberal just hears homophobia, but the conservative really believes that marriage is a holy institution, ordained by god, the basis of social order and that it needs protecting. Plus, he's a homophobe.
Society isn't purely atomic or molecular, but somewhere in between, with each person more or less strongly associated with the people around him. The ties that bind are strong only because we reinforce them with constant association; if you don't see someone for decades, it doesn't matter if he's a second cousin twice removed or your twin, he's become a stranger. By the same token, it doesn't matter if you're a WASP scion of a wealthy house and he's a farmer from Batang, Batang, if you spend hours together every day, you'll become close no matter how much you hate each other. In other words, we're neither a collection of atoms nor a tightly knit sweater. We're more like mayonnaise.
Homosexuality isn't a sin. It's not evil nor depraved nor harmful. To this extent the conservatives are correct: gays form friendships and families the same as anyone else and help us bind our society together. They're entirely incorrect because gay marriage wouldn't weaken the society, but strengthen it, confirming the ties that bind a significant portion of our population.
The conservative hatred of gays is an old one. It's codified in their bible, but I don't believe it began there. Homophobia is a disgust reaction, rather than fear, indicating that it's a social construct. In other words, I believe it's a combination of pillars three and five. Loyalty displayed through adherence to social mores, betrayal of which elicits a disgust reaction in accordance with sanctity/purity.
Iron age Christians distinguished themselves from the Greek and Roman cultures that were colonizing the middle east. It's even older than that for bronze age Judaism, as there were other cultures in the area that practiced homosexuality, even as part of their religion. The ancient Hebrews were notoriously anti-sex, which unhealthy obsession was passed down to Christianity. In other words, I believe that modern homophobia is a relic of ancient Jewish racism (which is also exemplified in such laws as "Thou shalt not wear a garment of divers sorts, as of woollen and linen together.").
Thus in-group loyalty was encoded as "avoid the gays from over there", which then was vested with sanctity and required to be maintained perfectly. And it was, for thousands of years, to the detriment of all.
So there it is, the ironic twist at the heart of the gay marriage controversy. Conservatives want to maintain the strength of society by fighting that which would strengthen it. And the cruelty that they espouse is not at all ironic; in moments of honesty they may even admit they hate gays, and are proud to still be practicing 5000 year old racism. That's one hell of a legacy.