Being a conservative, I advocate limited government. Big government leads to big trouble as we fight endlessly, acrimoniously, and fruitlessly over all sorts of issues that we really ought not be fighting over. As one of my slogans has it, "The bigger the government, the more to fight over." The final clause of the First Amendment of the U. S. Constitution enshrines the right "to petition the Government for a redress of grievances." So the more the government does things that grieve us, by intruding into our lives and limiting our liberties, the more we will petition, lobby, and generally raise hell with the government and with our political opponents. If you try to tell me how much soda I can buy at a pop, or how capacious my ammo mags must be, or how I must speak to assuage the tender sensitivities of the Pee Cee, or if you try to stop me from home-schooling my kids, or force me to buy health insurance, or force me to cater a same-sex 'marriage' ceremony, then you are spoiling for a fight and you will get it. Think of how much time, energy, and money we waste battling our political enemies, working to undo what we take to be their damage, the damage of ObamaCare being a prime example.
So if you want less contention, work for smaller government. The smaller the government, the less to fight over.
Along these lines, one might think it wise to sidestep the acrimony of the marriage debate by simply privatizing marriage. But this would be a mistake. There are certain legitimate functions of government, and regulating marriage is one of them. Here is an argument from an important paper entitled "What is Marriage?" by Sherif Girgis, Robert P. George, and Ryan T. Anderson. (I thank Peter Lupu for bringing this article to my attention.)
Although some libertarians propose to “privatize” marriage, treating marriages the way we treat baptisms and bar mitzvahs, supporters of limited government should recognize that marriage privatization would be a catastrophe for limited government. In the absence of a flourishing marriage culture, families often fail to form, or to achieve and maintain stability. As absentee fathers and out-of‐wedlock births become common, a train of social pathologies follows. Naturally, the demand for governmental policing and social services grows. According to a Brookings Institute study, $229 billion in welfare expenditures between 1970 and 1996 can be attributed to the breakdown of the marriage culture and the resulting exacerbation of social ills: teen pregnancy, poverty, crime, drug abuse, and health problems. Sociologists David Popenoe and Alan Wolfe have conducted research on Scandinavian countries that supports the conclusion that as marriage culture declines, state spending rises. (270, footnotes omitted.)
A very interesting argument the gist of which is that the cause of limited government is best served by keeping in place government regulation of marriage. A libertarian hard-ass might say, well, just let the victims and perpetrators of the social pathologies perish. But of course we won't let that happen. The pressure will be on for more and more government programs to deal with the drug-addicted, the criminally incorrigible, and the terminally unemployable. So, somewhat paradoxically, if you want a government limited to essential functions, there is one function that the government ought to perform, namely, the regulation of marriage.
For a different view, see this from Rand Paul.