An example of the problem: Pornography was originally against the law because of its lewd and perverse nature. This was based off of society's Judeo-Christian traditions. However, that same Constitution that we Mormons are destined to protect is what arguably* allowed for pornography to become legal. How do you "stop" same sex marriages? How do you stop abortions? How do you stop society in general from morally going off the deep end?
I fear that as much as we would like to say otherwise, there is no way to stem the "erosion of values" without laws enforcing those values. So instead of people being righteous on their own, we must compel them to be righteous. But is that the point of the Constitution? To force people to behave the way you believe they should?
So what then want was the point of the Constitution? The Constitution merely provides the framework upon which the structure of law-making government rests. Most of the Constitution therefore isn't really under attack, as many people say, because the Constitution doesn't stipulate any laws that people disagree with. No one disagrees with three branches of government or the powers of those branches. In fact, the Constitution doesn't even go so far as to stipulate what roles government in general should have, only what the Federal government should have. The only real contention that people have is what the limits of the federal governments should be, though most of that argument is conjecture, since Abraham Lincoln's power grab in 1860. If you really think about it, Abraham Lincoln destroyed the Constitution as it was originally designed by overstepping federal bounds set forth in the Constitution, by making a logical assumption that any one state of the union does not have the right to secede by themselves. That assumption, and the subsequent war that ensued ensured that no matter what, the Federal Government was the boss (that, and the 14th Amendment). That pretty much ended the discussion on federal limits on power.
What does all of this have to do with our conversation?
People often view the Constitution as being under attack by people who are trying to destroy societal values. The truth is, however, that there is not much morality on the Bill of Rights (the only guarantor of personal freedoms from the federal government). The only real moral clause is the eighth one banning cruel punishment. Everything else has to do with procedural issues on the role and limitation of government power in personal life. Therefore, as society becomes more wicked, the Constitution through the Bill of Rights is what is allowing these people to be wicked. Suddenly, what people never would have done before in their personal life cannot be legislated against without violating the Constitution and the personal freedoms it guarantees (like pornography).
You might have a problem with the federal government interfering with personal rights, as long as they cause no ostensible harm to others. The government does this all of the time though, and we think nothing of it. Prostitution, at-home drug use, and euthanasia are just a few examples of the government telling us what we can or can't do in our personal lives, and most people don't say a word in protest. Our objections to these behaviors are rooted almost exclusively in religious traditions. Most of our objections to government power have to do with money, not values.
Basically, using law to enforce moral values is not necessarily in step with upholding the values and freedoms established by the Bill of Rights in the Constitution. They can and often do conflict. Now I absolutely believe in being politically active as the Church suggests. But notice the Church never specifies that how your personal political beliefs should integrate with your spiritual ones. The fact of the matter is that most people's political beliefs contradict their religious ones, and they would rather not face that ambivalence, so they ignore the discrepancies.
I look forward to your response.
A Note: In quoting Ezra Benson, we must remember the context in which he speaks. Benson was an outspoken anticommunist, a fact which had many members of the Church worried that he would say something politically stupid from the pulpit. He didn't ever really have too much of a problem, but his personal political beliefs came through on many occasions. It doesn't mean he was wrong, but his opinions on politics have just as much bias as Joseph Smith's, who at the time suggested we dump all the black slaves into Mexico, since they were of similar skin color.
*The First amendment allows for the freedom of speech, press, assembly, and it does not take a far jump to say that words, images, and graphics, granted they do not ostensible harm or present any clear and present danger, are protected by that clause. As repulsive as they may be, without injecting moral opinion into the Constitution, there is little way for it to say otherwise.