You are right.
The institution of marriage is something not instituted by man, but by God. (Genesis 2) It will survive even the outrages we are now witnessing for the very reason that it is an institution of God. It won’t go away. It is what it is, even if the majority within a society abandon it and, thus, deny its blessings to themselves and their progeny. The society itself will come apart, but the institution will long outlast it, as history has proven many times.
On the other hand, marriage is something properly administered by the state, i.e. government as opposed to church. By this I do not mean the state can do with it as it pleases or define it according to current tastes and trends. Again, it is what it is, and the church knows better than the state the truth of that. The state can only administer it as it was originally instituted and for the purposes it was originally given: 1) lifetime partnership/companionship/love and 2) propagation and preservation of the human race. Any honest historical examination of the laws of the 50 states (since this has always been a matter of state rather than federal law) will show this to be true.
It is no coincidence that government, in failing to fulfill its proper role in upholding this, the 6th Commandment, is also failing to uphold the 4th Commandment (protecting the honor due parents from their children), 5th Commandment (protecting the sanctity, i.e. the God-giveness, of life), 7th Commandment (protecting the right of ownership), and 8th Commandment (protecting the right of reputation). In other words, we are witnessing the breakdown of our society, something which has happened to many nations before. When the law is used to protect vice at the expense of virtue, the slide down the slippery slope has begun.
“On the other hand, marriage is something properly administered by the state, i.e. government as opposed to church.”
But if the state is involved, how do you avoid eventually conditioning people to think the state defines marriage? Seems to me thats really the big reason so many accept impossibilities like gay marriage: it can exist because the state decides it can. You even see many faiths only do gay marriage ceremonies if the state gives its permission, even though that particular faith might already think gay marriage is possible, but wont act before the state decides it is officially.
Freegards
God cannot be pleased.