The notable silence by the Republicans on the issue of same-sex marriage during the current election campaign is a sign, if one needed one, of how rapidly sentiment has shifted on this issue. In 2004 opposition to this was very potent and was used to galvanize voters to go to the polls and vote for George W. Bush. Daniel McCarthy argues in The American Conservative that it is one more sign of the retreat of religion in the face of modernity...
Even conservative activist Phyllis Schlafly thinks that they are losing this battle and blames (of course) public education.
No. You are losing the fight because you are a bunch of backward, bigoted, troglodytes and people don't want to be associated with you.
There is an extensive quote from an article in The American Conservative, and the author tries his best to frame the loss of a fundamental tenet of conservatism as something that is totally cool. I guess he is tired of being automatically classified as a bete noire at dinner parties when everyone there finds out he writes for a magazine called The American-freaking-Conservative.
My recommendation: stop being conservative. Then you won't simply be the most recent iteration of a grand tradition of being left behind by the progress of society.