Marriage is defined as between a man and a woman.
Definitions are not rules. They aren't laws. A definition is a description of what a word means at the time the description is written. It is in and of itself amoral. It is the consensus of what speakers of the language mean when they use the word. Therefore unless we clarify what type of definition we're referring to (legal, scientific, etc.) the definition changes as quickly as our usage. So what the word is documented as meaning cannot be an argument against change. The fact that the definition would have to change is not persuasive in any way since they'll print a new edition of the dictionary next year if the definition of marriage evolves or not.
Secondly, we only disallow marriages based on gender. Any other infraction of the definition of marriage in society today does not lead to a politically required banning of or end to a marriage.
Adultery does not affect an immediate divorce in any state... the partner must file for it. If two people define their marriage as open (meaning they are free to be sexually active outside the marriage) then we still allow them to marry. Even though a monogamous homosexual relationship is probably much closer to the traditional definition of marriage.
If we say gay marriage is acceptable, then next we'll say that (enter awful sounding sexual act usually involving childern or animals) is ok.
This argument is wrong for two reasons. First its an argument based on the slippery slope fallacy. Simply, that one action without a doubt will lead to a secondary action which in turn will lead to another. In its purest form the fallacy is called the Armageddon argument, where whatever action someone is arguing against will without fail lead to the end of the world, whether literally or figuratively. For an argument to retain any validity it must make a direct causal connection between the primary action and the caused effect. The argument that gay marriage will lead to inter species marriage is my personal favorite. Heterosexual marriage has been around for some time, but a man marrying a female horse I'm fairly sure is still illegal. The point is this. The only place there is a similarity between gay marriage and these deplorable actions that opponents bring up is their own distaste for the actions themselves. There is no logical reason why gay marriage would lead to any other type of contract that doesn't specifically have to do with homosexuality.
Secondly, this argument sometimes finds an audience because of a general misunderstanding of homosexuality. There is no direct tie between homosexuality and actions that we would consider unacceptable or sexually deviant. Pedophilia is as horrifying to the homosexual as it is to the heterosexual. The basic core of homosexual or heterosexual marriage is love, commitment, and consent between two people.
The Bible says that its wrong.
If people don't believe in the Bible then this is just a stupid argument. You're presupposing that something gives you credibility that doesn't and it just makes you look dumb. Even if you can completely prove that the whole Bible undeniably condemns homosexuality (which is doubtful) then all you've proven is that your opponent disagrees with the Bible and thwarted your own ability to use that text as a source of wisdom or inspiration on any other subject. Think of it this way... If you think the Bible is a moral compass and I argue that something is wrong because the Koran says so, is that really helpful?
The Happy Argument
Version 1 Gay marriages aren't happy. I know of one that is... argument over.
Version 2 Most Gay marriages aren't happy. Most heterosexual marriages aren't happy... next?
The Pastoral Urbanite