Anonymous asked for a definition of marriage, and, as it seems like a reasonable request, I'll give mine.
There are two aspects of marriage to consider. One is a sacred contract sealed between two or more people in the presence of family and their God. There is no legal aspect to this – it is simply a promise, made in the presence of God to seal it with holy sanctity. The details of who qualifies is determined by religious hierarchy. Some religions endorse polygamy, some define marriage as between one man and one woman, some endorse same-sex marriage. All of these religions can be found in the United States, and all of them currently practice marriage by their definition. In a free society, each religious tradition defines marriage as it sees fit; there is no obligation to treat different configurations equally, or even to recognize that they exist.
The other is a legal contract, made between two or more people in the presence of the state. It is an agreement on dispensation of property, ownership of children, and social organization. The role of a government of free people in regulating this contract is, first, to ensure that all parties are consenting adults – a consenting adult being someone who has reached the age where they fully understand the obligations, responsibilities, and consequences of joining into this contract, and who is entering into it of their own free will, without coercion. The second is to apply the law evenly to all citizens, which means that once assured that all members of the union are consenting adults, the state allows them to sign the paper joining their individual lives, and treats them henceforth as one unit within the bounds of the marriage contract.
Yes, that means I believe the state should allow same sex and multiple partner marriages, so long as all members are consenting adults. The state also has a duty to prevent marriages that include young people under the age of consent, people who are under pressure from family or church, and people who are incapable of understanding what they are doing. And yes, this is a very libertarian philosophy or definition, but that doesn't make me a libertarian. I just believe in freedom, responsibility, and conscience. I believe in the principles under which this country was created.
Note: I did not consult a dictionary in writing this. Anonymous asked my opinion. Here it is.