There are churches today perfectly willing to perform same-sex marriages. Prohibiting it denies to these churches the right to practice their religious views as they see fit. To oppose gay marriage is to behave anti-individualistically and collectivistically - to be a socialist-of-the-spirit.
There is no difference between saying "the State should have the authority to direct the resources of individuals as it sees fit" and saying "the State should have the authority to direct the private lives of individuals as it sees fit". None at all. And these anti-communist conservatives who oppose gay marriage are but mirror images of the Statist authoritarians they pretend to rail against; they have simply abstracted their collectivism to the level of the 'spirit'.