I also support same sex marriage.
I think the "slippery slope" argument is a cop-out. It's either right or wrong or you're undecided, but IMO an issue needs to be decided on it's own merit, not what you might foresee somewhere down the road.
For those who are against same sex marriage based on what it may lead to, do you also think that interracial marriage should never have been made legal? The similarities between that issue and this one are often pointed out, so some could say that Loving v. Virginia started us down the slippery slope that will lead us to same sex marriage, polygamy, and people marrying their goldfish.
Nope. Each possible new expansion of the definition of marriage should be looked at on it's own. As ZephryHawk explained, there are many reasons polyamorous unions don't fit into the marriage mold as well as same-sex unions do, and those should be analyzed if and when the time comes that the legality of such unions is questioned. Not as part of the issue of same sex marriage.