Of course they do, if by "liberty" you imply absolute liberty, which is anarchy. You know this, and the "liberty" gimmick is employed anytime a leftist liberal doesn't get his way. He isn't concerned that liberty is being limited, only that his particular wants are being denied. We live under a penal legal system, which effectively says that all behavior is permissible unless prohibited by some specific law or ordinance. So, every law passed does in some way limit what an individual would otherwise be free to do. This is acceptable in a society in which millions of separate egos operate - it always has been acceptable. Otherwise, the extermination of mankind is assured.
And what's the criteria that determines what behavior is restricted or limited? That's what the democratic process is designed to do. And this is based, as least historically in the United States, on a Judeo-Christian moral code, and by a consensus of what the majority of voting citizens determine is acceptable behavior and norms, either directly or through elected representatives.
I think the chances of the marriage amendment being approved by the public is somewhere between slim & none...
You mean like the numerous marriage initiatives introduced within the last several in the vast majority of states falling flat? I think you need to refresh yourself on the facts.
...the only way this could ever become law is by decree.
Who's decree? By 4 judges? Democrats and liberals know this method only too well.
I don't think that the "gimmick" is limited to just liberals. It is doubtful though that your logic will be successful in preserving marriage because your own agenda gives you away.