I've said it before and will say it again... The only arguments against gay marriage are religious ones which hold no merit for a variety of reasons. First of all, the arguments stand and fall on their own. No part of the bible forbids gay marriage, explicitly or even ambiguously. Leviticus contains the sole passage which commands to execute two men who sleep together for committing an "abomination". Nobody follows this anyway in the civilized world, fortunately. So why merely oppose gay marriage if you're not going to become the hitman this holy text compels you to become? Besides, stopping gay marriage doesn't preclude them from doing the do, if you catch my drift. And why people even spend a moment of their day concerning themselves with THAT, I'll never understand. Now, we also have this long-standing American principle of separation of church and state. When state workers such as that obese woman who is currently justly jailed for violating two court orders to do her job and issue marriage licenses to a gay couple despite being divorced multiple times herself and therefore requiring a marriage license multiple times, these people are spitting upon American values, interests, and citizens AND failing to represent their religion. Laughably, societal concerns have risen from deluded conservative minds such as economic implications in allowing gay marriage to occur but of course this is fluff and laughed at by actual economists.
TL;DR: Hence, if you are against gay marriage you are an inexcusable, un-American reprobate and a failure within your religion as well. End of story.