I'd like to add to what Jonathan has written below, on allowing religious people "conscientious-objector status" when it comes to requiring actions that affirm the equality of same-sex unions.
Almost all gay people, I'd say, want to be treated equally by the government, with the same rights and responsibilities as all citizens. That includes the right to marry (even if they choose not to marrry) and, for most, the right to serve in the military (even if they would choose not to do so).
Some gay people, however, don't merely want equal treatment by the state. They want to use the state against those who, based on deeply felt religious belief, do not want to offer their services to same-sex marriage or civil union ceremonies, as Jonathan describes below.
That's called progressivism, but others would say it's engaging in a legal vendetta against those who hold religious convictions that run counter to the principles of gay equality.
Another example that has garnered much publicity is from Canada, where an anti-gay pastor is appealing his conviction for writing a letter to a local paper that was found to defame gay people (who were compared to pedophiles and drug dealers), and thus to have contributed to a climate that fosters anti-gay violence.
The U.S. religious right is having a field day with this action in Alberta, charging that it's a reason to oppose measures such as the proposed federal Employee Non-Discrimination Act. And that, in turn, has led some supporters of gay nondiscrimination to defend the Alberta ruling, holding that speech that incites ill will should be banned.
But that is indeed a slippery slope, and one that runs counter to the right to express unpopular, and indeed ugly, opinions - a principle once defended by liberals.
More. Dale Carpenter, writing over at The Volokh Conspiracy, shares his thoughts on religious liberty and same-sex marriage. Excerpt:
Religious freedom is a first and founding principle of this country. I think religious accommodation to private persons and organizations should be generously provided, even where not required by the Constitution. At the very least, accommodation should be made where it can be offered without harming the protected class. For that reason, I think an exemption should have been offered in several of the cases cited in the NPR report....
While I'd be generous about accommodating the religious objections of private persons, I am very wary of introducing a system of exemption for public officers serving the public with taxpayers' money.