Who should have authority to determine who can and cannot be married? The Goverment, certain religous groups? Does anyone really need the institution of marriage? If a couple (or more, polygamists) love each other, want to live together, start a family together can't they take resposiblity for their own lives, actions and relationships?
Well the basic answer is that if you live in a country where it is considered by society enough that a couple, group, human/tree or whatever love each other, want to live together, want to start a family together and want to take responsibility then I don't think it's appropriate for the government to stand in their way and prevent them from enjoying all the benefits that same government affords actual, real marriages. Though enjoying the same benefits - that doesn't actually make any of the above perversions actual marriages, only God makes marriage. Societies and governments can only recognize that they exist or refuse to.
God decided what marriage
is and He decides who is married. 1 man 1 woman united by the 1 God. <- that's marriage.
But regarding regarding whether government or society is the proper arbiter for deciding which unions should receive social and legal benefits (like inheritance, medical coverage, right to visit each other in the hospital, privacy),are legal questions which should be decided by the culture/society/people and not its government - still that does not mean society is decides what is marriage. Just like before, God already decided that.
A piece of paper from the government telling me I'm "married" because they said so about as ridiculous as receiving a piece of paper telling me I'm "dating," "single" or "married to my work."
After becoming married to a single, unmarried, person of the opposite gender in a Church (the only way a person has entered into a marriage) if my marriage is considered one of the "recognized legal unions" it is a proper function of goverment acting on behalf of the will of society to provide me and my spouse with rights to visit each other in the hospital or to collect inheritance.
So if a society wants to consider gay unions, polygamous unions or any other sinful attachments something worthy of recognition for legal rights then have at it. It doesn't make it marriage, only God does that.
What frightens me is when people start worshipping their governments in the hopes said governments will solve all their problems - that's where you get idiotic ideas like "Gay Marriage" a contradiction in terms or amendments to ban "Gay Marriage" ... which is a bit like banning 10 divided by 0. Either way both parties are asking government to define marriage.
That one's already been defined.