The problem is an out-of-control Supreme Court which legislates from the bench rather than interprets the plain text of the Constitution.
The quick answer is probably impeachment, but I don't see that happening. The next best thing is getting strict constuctionists on the Court, but the Dems are intent on blocking that and they probably will succeed.
Fundamentally, the country needs a massive revival of Constitutional understanding and principle, or it is lost to the eroding tyranny of the creeping socialist state. Revival can only come from the grassroots and must result in the complete overhaul of our education and media to have any lasting effect.
The country is split down the middle with more statists coming in or being produced by the education system every day. Something has to give if we are to live peacefully together! At some point, those determined to uphold the Constitution will inevitably say "enough". What will happen then is very much up in the air.
Here's the really quick answer:
Article III, section 2: "In all the other Cases before mentioned, the supreme Court shall have appellate Jurisdiction, both as to Law and Fact, with such Exceptions, and under such Regulations as the Congress shall make"
Congress can remove appellate jurisdiction from the Supreme Court over state laws concerning marriage, or even, in Mr. Justice Kennedy's memorable phrase, "matters pertaining to sex". They could do it in a week, by simple majorities.
This is much easier than amending the constitution, or impeaching justices.