Last Friday, after the Supreme Court ordered everyone to get gay married, Duncan "Atrios" Black at Eschaton asked a good question: why do they care? Specifically, why do conservative Christians spend so much time and effort angsting over gay marriage? "No one is making you get gay married. No one is telling your pastor he has to have God bless this legal arrangement established by the state." Black figures that they're running out of people to hate, and without someone to hate their lives will have no meaning.
I think the answer is a lot more evil and cynical and terrible than that. To find out why, you'll have to go back in time about forty years to the Disco Era. Some conservative Christians back then decided that what they really needed was a lot of political power, and the way to go about getting it was the usual way conservatives go about getting political power: find an unpopular minority group, and demonize the fuck out of it.
So these conservative Christians gave the matter some thought, and they finally settled on homosexuals as the perfect group to Hitlerize. For the next forty years, they devoted a lot of time, effort, and money to the project, and they succeeded. The result is what we have now: a whole generation of conservative Christians, tens of millions of them, who have been brainwashed into believing that homophobia is the core of Christianity.
In vain will you point out to them that Jesus didn't so much as mention homosexuality. They don't care. Forget about that whole Jesus dying on the cross thing. As far as they're concerned, the REAL point of Christianity is hatin' on the gays. That's why, whenever they talk about "religious liberty," it refers exclusively to discriminating against homosexuals. Deny them their sacred right to discriminate against LGBT's, and they act like you're feeding them to the lions in the Colosseum.
And that, to answer Black's question, is why they care. Because they've been raised to follow a cynical, evil creed that replaces worship with an Orwellian Two Minutes' Hate.
So how will this turn out? It seems to me that the congregation of the Church of Homophobia have three options: some will decide that the Christianity = homophobia equation is false, and they'll find their way out and back to mainstream Christianity. Some will decide that the equation is true, but that homophobia is wrong, and they'll reject both. And some will stick to it to the bitter end, part of an ever-dwindling minority drowning in their own hate.