We have a 50% divorce rate. That may make your point, but obviously marriage isn't all that sacred to a lot of people. That's one reason I'd like to see it back to the churches.
2) The moral depravity that the homosexual idea represents.
A lot of things have moral depravity. Should everything immoral be illegal? If so, how is it enforced?
3) The fact that if you give government over to moral depravity you will have destroyed the possibility of republican, constitutional self-government.
Government is at best a necessary evil in and of itself. It has long been given over to moral depravity. How many people have been killed by governments throughout history?
If marriage is not defended on every single front, including the national level, the demise of the republic is assured, whether folks will recognize it or not. This particular institution is that fundamental and indispensable.