I'd like to recommend a book called "What Is Marriage For?" by E. J. Graff. It's openly biased towards gay marriage, but it's very informative and does a good job of separating the opinions from the facts.

And I'd like to second just about everything Aesop has said so far, it would take a bit too long to address every point individually though.

Marriage is much more than a formality. It enables two people to easily manage their shared property, represent each other legally and publicly, make important medical decisions, et cetera.

I'd also like to point out that, once gay marriage is guaranteed legal, there will be a lot more people getting married initially- but not just because it's a novelty, but because there are so many people who have been partners for a long time and have been waiting to get married. There would be an initial rush because there's a long line outside the door!

One last thing. Leading a sub down the street on a leash is no more sexual than two people walking down the street holding hands. Does it hurt a child to see a couple holding hands? Does it hurt them any more if those two people are the same sex?

That's all.