conservatism, same-sex marriage, and small government

"I like you guys who want to reduce the size of government - make it just small enough to fit inside your bedroom."   Joshua Lyman, The West Wing, Season 2, episode 6

Conservatives love saying they want smaller government. Take this line from the mission statement of the National Review:

It is the job of centralized government (in peacetime) to protect its citizens’ lives, liberty and property. All other activities of government tend to diminish freedom and hamper progress. The growth of government (the dominant social feature of this century) must be fought relentlessly. 

I like this idea. But I'm afraid conservatives don't take their own mantra seriously enough. William F. Buckley's idea that the government's sole job is to protect lives, freedom and property, is a modern refrain of a much older idea, perhaps best put to words by John Stuart Mill: 

the sole end for which mankind are warranted, individually or collectively in interfering with the liberty of action of any of their number, is self-protection. That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others. His own good, either physical or moral, is not a sufficient warrant. He cannot rightfully be compelled to do or forbear because it will be better for him to do so, because it will make him happier, because, in the opinions of others, to do so would be wise, or even right. These are good reasons for remonstrating with him, or reasoning with him, or persuading him, or entreating him, but not for compelling him, or visiting him with any evil, in case he do otherwise. To justify that, the conduct from which it is desired to deter him must be calculated to produce evil to some one else. The only part of the conduct of any one, for which he is amenable to society, is that which concerns others. In the part which merely concerns himself, his independence is, of right, absolute. Over himself, over his own body and mind, the individual is sovereign. (Mill, On Liberty)(emphasis added). 

The idea here is called the harm principle. Basically, don't restrain me except to protect others, and my own virtuous or not-so-virtuous lifestyle is not your concern. Conservative friends, how does this idea affect your views on same-sex marriage?