I've been quoting this, and since it's now PAGES back, I want it posted here so I can find it quickly:

To the Progressive Socialist Totalitarian Left, Christianity is a threat to the primacy of the State. The Totalitarian Left believes the Authority of the State must be absolute, because the left can control all the apparatuses of the State and impose their moral beliefs on the population. For example, the belief that unborn children can be sacrificed in the name of personal convenience and the sick and elderly can be sacrificed to save the State money. Christianity, on the other hand, teaches that there is a Higher Moral Authority than the State; and that the conscience of the individual… not the Collective Will as embodied in the State and its organs.

It isn’t necessarily because of Gay Marriage, per se, but Gay Marriage is a cudgel that the left can use against Christianity; forcing Christians to bow to the State (e.g. being forced to participate in gay weddings as bakers, photographers, and florists). The ultimate goal is to eradicate Christianity and its tenet that each individual has a conscience and a moral imperative.