They would not be agreeing with you.
A lot of truth to what you say here. Libertarians make some good arguments, but I do think it's fair to say that the people who helped build this country would never have supported something like normalizing gay marriage. I understand the libertarian points about government staying out of these issues, but I suspect the founders, even as committed to personal freedom as they often were, would not have been willing to give up the culture to this sort of thing.
Then again, the country would probably have also strung up a party that wanted to free blacks or give women the right to vote in 1790 too. There is an argument that the times have always changed and we have to be willing to change with them. To me though the issue is this - if we lose the culture to total degeneracy, is there really anything left fighting for? And if the government starts walking away from these social issues on all fronts (which may be the road we are on), where does it end? Will libertarians be okay with people marrying and having sexual relations with their pets? How about polygamy? How about incest between consenting adults? Where does it end?
These are all great issues for the states to consider, NOT the federal government. That role was clearly and unambiguously given to the states.
Where in the Constitution do you find the federal government given the power to define marriage? The states were given that authority. That the feds have claimed unconstitutional power over things affected by the definition of marriage makes no difference. The federal government does not have the authority to do it.
I understand that you would find it easier to grant the federal government the power to define marriage, so long as that definition is agreeable to you. But then, with you claiming they have that power, the feds change the definition to one that is clearly ridiculous. But YOU gave that power to the feds. THAT is the libertarian complaint.