Terry Pratchett, through the character of Sam Vimes, notes that all crime is theft. Burglary and theft and embezzlement are certainly theft. Kidnapping is theft of a person. Murder is theft of life. Other crimes are theft of privacy, from simple trespass all the way up (or rather down) to rape. Plagiarism is theft of ideas. It is worth asking then, when something is to be considered criminal, what it is that is being stolen. Not what might be stolen by error or abuse, but what is being stolen.
If one takes the time to examine the founding of the United States of America and its constitution, there was really only one crime, one theft, that had to be guarded against: the theft of individual liberty. What government can do is carefully limited. Some of what government is barred from doing is listed, but it is noted that the listed limitations are not necessarily all of the limitations. Where there is question, the answer is this: Do what maximizes individual liberty.
Looking at the issue of marriage for homosexuals, can anyone show anything being stolen? Despite cries from some religious folks that such a thing would harm heterosexual marriage, there is no evidence of how this could be. Would homosexuals suddenly start stealing wives or husbands? The idea is absurd. Perhaps it is meant as stealing from the pool of available men or women - yet by the very definition, the pool that gays and lesbians would take partners from is not the same as the pool of available heterosexual partners. As there is no theft, where is the crime? And, which way lies greater individual liberty?
Nobody has to like the idea for themselves, as many no doubt dislike the free choices of others. People disagree on who to vote for, on what church to attend, whether to attend one at all, what do with their money, and on and on. But that is the beauty of liberty: it doesn't matter what others believe. You choose whether to vote, and if you do, who to vote for. You choose whether to participate in any religion, and if you do, which religion. You choose how to spend, or save, your money. Already millions disagree with you about which way, if any, to vote, what religion, if any, to pay heed to, and the best thing to do with your money. But that's not a problem, unless they make their own choice to have heartburn over choices that are not theirs to make. So why shouldn't you be able to choose who to marry? Because it might piss someone off? What say should anyone not directly involved have in what is your choice? None at all.