On Slippery Slopes and Polyamory

In my last post I argued that same-sex marriage is not a new phenomenon but, rather, the act of applying an old institution to couples who have historically been denied access to it. In an interesting comment on that post, blannphinella asked whether the marital kind of relationship can also be had by groups of more than two.

Put another way, does conferring a marriage on parties of three or more people involving simply making marriage as ordinarily conceived available to more seekers than have historically had access to it? Or does "marrying" a group of more than two necessarily involve changing the kind of relationship at issue? Can a triad possess a "marital" relationship that is the same in kind as what a pair possesses?

Now this question is important, because a common challenge to marriage equality is that it is a slippery slope to polygamy/polyandry. If we make marriage available to same-sex couples, then what's to stop us from making it available to groups?