As it seems the GOP has lost most of their favorable rights and traditions in marriage equality. The man and woman only marriage seems to drift away to a new era and the new generation growing up will also be more favorable and tolerant to the idea of the same sex marriage.
President Obama is the first President to come out fully in support of same sex marriage and he is now going to be an example for future Presidents to follow. Saying that marriage equality should not be a thing is only going to be seen as taking steps backward. The fight for marriage equality is moving, maybe not a rapid pace, but it is steadily moving forward and there really isn't much more that the GOP can do or argue without stepping on too many toes, and they need to be careful of that and rebuilding their favor with voters for the next election rather than alienating them.
The reason being is is balance and disagreement. The anti gay republicans have certainly lost the battle. Everyone seems to have different beliefs and this is certainly a touchy subject. With different agendas and more confusion than a insane person's brain the GOP has lost the fight.
Most states still favor traditional marriage. Only some of the most extreme liberal states, like Vermont, have a majority of people who support same-sex-marriage. Heck, even in California the majority of voters voted against allowing same-sex-marriage. It had to go to the courts to be overturned. More than half of Americans still oppose it.