It's true. Why, just a few years ago, in 2004, George W. Bush used "gay marriage" as a wedge issue to win Ohio and thus reelection. Now, the president of the United States, over 50 senators and the majority of Americans approve of "marriage equality." No matter what the Supreme Court decides on the matter, the country has clearly "evolved."
The same is true with immigration reform. Although it may prove to be a bit of an issue in the 2016 Republican primaries, Sen. Marco Rubio, the tea party darling and establishment candidate (so far), has endorsed the current legislation. It's expected to pass soon with bipartisan support. Again, evolution.
As for guns, well, we're not quite there just yet. Although fewer and fewer Americans own more and more guns each year, the country is clearly evolving on the issue. And while the gun safety legislation currently before congress may or may not succeed (and it's probably toothless anyway), American opinion on the subject is evolving, if ever so slightly (the NRA lost a vote last week). What the gun debate needs most is time. It could take a few more generations, but I suspect that Americans will evolve on gun safety just as they've evolved on gay rights and immigration.
I guess we just have to be patient.