Federal constitutional amendment? No
Banning it on a state level? Yes
Not a very libertarian position there.
Sure it is, I want the state government to exercise its power, not the federal government. You can argue that perhaps limiting marriage is not libertarian, but the idea of libertarianism is to stop the flow of government, not to lead the country into anarchy. Sometimes government is necessary, but in a very limited and not federal form. To have a personal opinion is quite different then forcing it down the entire countries throat.