You wouldn't have a Republican Party without traditionalist conservatives.
What is "conservative" and "traditional" changes throughout history and cultures. Gay marriage is now increasingly accepted, and soon will be seen as normal.
The Republicans can still keep SoCons in their tent based on abortion, guns, affirmative action, and a wide range of other social and cultural issues, but opposing marriage equality is a lost cause that will only drag the GOP down if they keep fighting it. They can afford to drop opposition to it (slowly, though), but not on the other issues, which is why people who think the solution for Republicans to become "socially liberal but fiscally conservative" are fools. Gay marriage is the only issue they're badly losing on, everything else is a 50-50 debate.