Yes, Republicans are shameless flip floppers who base all of their positions on the potential for political gain, while Democrats who do the same thing (ahem, Mr. President) merely experience an evolution of their opinions.
I think the difference is that Republicans were actively pushing for bans on gay marriage, constitutional definitions of marriage as between a man and a woman, etc while the Democrats were largely just quiet on the issue. It's easier to forgive people who were just passively neglecting you rather than actively trying to shore up discrimination. Obviously not all Republicans were on board with the social conservative ideology, but there's the guilty by association factor.
It's honestly amazing how quickly the gay marriage issue has changed. As recently as 2008, there was a tide of gay marriage bans, and even prominent Democrats (like Obama) were not pushing for gay rights and talking about their own personal reservations towards them. Now we have 11 states with gay marriage and just a stampede to get on board with "evolved opinions." Has there ever been a major social issue where the prevailing opinion changed so quickly?
I wonder how much of this is a backlash to the anti-gay push from the social conservative side circa 2008.