There are three arguments generally used by the right when attacking gay marriage. None of them are very good, but since we keep hearing them - and hearing people do a poor job of refuting them - we're going to go through them one at a time.
1. Traditional marriage has always been between a man and a woman.
First, this is just factually inaccurate. Traditionally marriage, as in good ol Biblical marriage, was usually between one man, one woman, and then a number of other women captured as spoils of war. The Bible even has rules for polygamy, such as not treating your kids differently just because you like one mom and the other is a wretch (who can blame her, you killed her husband, kidnapped, and raped her). And speaking of rape, traditional marriage has been perfectly okay with marital rape, but we recognize that's not so great. Traditional marriage also didn't allow for divorce, but we're fine changing that too.
Second, the historical argument is completely unnecessary. Our nation was not founded on the idea that tradition trumps all. Traditional governments were monarchies and we clearly didn't buy into the argument that that government "is between one king and his subjects." God created David and his subjects, not David and his constituents.
2. Gay marriage devalues marriage.
No, shut the fuck up.
Letting anyone call their product "organic" without a set of standards devalues the label. Letting anyone call beef "Kobe" even though it was raised from the wrong breed of cattle and fed the wrong grains devalues that label. We need to protect those sorts of labels because consumers do not have the expertise or resources to investigate every product they're purchasing. If you're worried that letting gay people get married will somehow make people not sure what it means when you say you're married, then the problem isn't that gay marriage is devaluing marriage - it's that your wife looks like a man.
You could make a supply and demand argument for the devaluing. Right now there's a certain number of marriages, and a certain demand. Allowing gay marriage increases the supply. It does not increase the demand (the demand is there currently, if it wasn't no one would care about getting gay marriage passed). So, we're increasing supply while keeping demand the same - that's a recipe for decreased value. Of course, you have to believe that your marriage is some sort of commodity, and why do you care if it's devalued in the first place? It's not a car, it doesn't have a resale value.
3. Slippery slope, we're gonna let three men and dog get married.
Again, just shut the fuck up.
Slippery slope arguments only work where there's not an obvious place to draw the line. Consenting adults is a really obvious line. Now, drawing the line at two is a bit trickier, since three people can all be consenting adults. And of course, under the traditional marriage argument polygamy is just fine. So, we could make the argument that if we allow one man and one woman to get married, what's to stop one man and two women from getting married? We need to ban straight marriage to protect the sanctity of marriage!
The extreme end, the man-marries-dog catastrophe, is a non-starter because there's just not a slippery slope. Or, more exactly, it is a slippery slope than very quickly flattens out, and the man-marries-dog thing is a whole other slope with guard rails and warning signs around it.
As for worrying about plural marriages? So the fuck what? Arguing that gay marriage will lead to polygamy isn't an argument against gay marriage unless you can also put forth a good argument against polygamy, and "the taxes get too complicated" isn't enough. Our taxes are already complicated.
4. Bonus Fourth Argument! Free speech y'all!
Because the other arguments are so easily dispatched, let's just get a little crazies and see where this goes.
It should be illegal to oppose gay marriage. Why? Because the President has come out in favor of it, and you should not be allowed to criticize the policy positions of our head of state.
What? Free speech!
Oh, hell no. If we give you the freedom to just say whatever you want, you might start saying really bad things. We can't allow that. Traditional speech has always forbid criticizing the King. Letting you say dumb shit devalues the smart shit I'm saying. We're going down a slippery slope that will lead to saying all sorts of crazy things (My mother is a fish.).
Of course that's not how we (are supposed to) view freedoms in America. The fact that someone might exercise their freedoms in a way you find objectionable is not a reason to deny them their freedoms. That actually sort of goes towards what it means for something to be a freedom in the first place, that they get to use it however they want. That you don't want gays to get married isn't an argument against them having the freedom to do so, no more than saying that Matt and Trey will say no-no words on South Park is an argument against letting them do it.