Government capturing the word "marriage" is a reason "traditional marriage" is at risk to begin with.
No longer the mere recorder of committed couples, governments have enacted vast bodies of law conferring responsibilities and privileges to those it alone deems qualified to marry.
Qualifications vary from state-to-state. At times the rules have excluded interracial couples and even divorcees. The libertarian point is all couplings are not treated equally.
The same rationale that government has a right to encourage some "social goods" (some kinds of marriage), allows it to also discourage others (smoking, food consumption) and even create whole protected classes via hate crime legislation.
Libertarians want the most extremely limited government sustainable. Government defining marriages, rather than simply recording, most certainly is an expansion of its power, reach and authority.
Ron Paul suggests private contracts together with personal religious authorities be the determiners of who qualifies for the "married" label and the responsibilities and privileges thereof.
You can argue defining marriage is a proper role for government but you can't argue it isn't growth of government's power over individual lives and interpersonal relationships. It's at that level the libertarians bristle.
Government defining marriage in some way predates America, age restrictions, siblings, polygamy, etc. I’m not aware of a time in American history when government did not limit marriage.
The Government has every right to define what they accept as “marriage” They have No right to define “marriage” there is an important distinction between the two. The Federal Govt. via the supreme Court attempted redefine “marriage” back in 1987.Prior to that in Reynolds v. the United States the Court
said the Mormons could define Marriage however they desired but the United States would not recognize polygamy as legitimate religious practice. (Acting not on the definition of “marriage” but upon the actions of individuals associated with the Mormon Church.) In Murphy v. Ramsey and others (1885) the supreme Court defined “family” and by extension “marriage”as consisting of and springing from the Union for Life of one man and one woman in the the estate of Holy Matrimony. So the Government has been in the business-if you will of defining “marriage for a long time. But exactly how can you say defending DOMA will grow Government?How can defending the Founding Principles of Religion, Morality and Knowledge be indefensible. I cannot be a Libertine /or a Libertarian because they do not defend the Founding Principles.