To the extent "marriage" matters to government, it should on be the economic and parenting relationship that the government has any interest in. The concept of "marriage" itself isn't a government concern, or at least shouldn't be.
Unfortunately, red flag social issues - gay marriage, flag burning, abortion - tend to consume our politics when actually very little of anything of those issues change regardless of who is elected.
The gay marriage issue is becoming a real handicap to the Republican candidates, but one they are stuck with. Because "flip flop" has become a dirty word in politics, modifying their position is all but impossible. The notable exception is Ron Paul, but his lack of anti-gay talk put up a wall between himself and the religious right, which also has much overlapping with the Tea Party.
Even among those who oppose gay marriage itself, most are not vehemently anti-gay and don't like strong homophobic declarations - putting candidates like Bachman in a real box. But for her extreme social stances she would be a far more viable candidate now.
Herman Cain proved that Republicans really aren't the bigots they oft portrayed to be because 'Republican" and far-religious-rightwing and Tea-Party are not the same. The latter are only vocal fringes of the Republicans.
The gay rights movement has won. There is only the uncomfortable lag time between now and then that is accepted.